Kim on Causal Exclusion
[Draft: 7/22/02]
Brian Beakley
Philosophy Dept
Eastern Illinois University

"To appreciate the exclusion problem, we do not require much heavy-duty metaphysics – overarching doctrines about mental anomalism, "strict laws" in causal relations, a physical/mechanical conception of causality, token physicalism, and the rest.  It arises from the very notion of causal explanation and what strikes me as a perfectly intuitive and ordinary understanding of the causal relation."
-- Jaegwon Kim (Kim 1998: 66-7)

I. Introduction. In his writings over the last decade or so, Jaegwon Kim has come to focus almost exclusively on presenting a refutation of contemporary functionalist philosophy of mind – a position which he aptly labels “non-reductive materialism”.  According to such a functionalism, no non-physical objects or events are anywhere premised, yet mental properties do not reduce to physical ones.  Central to Kim’s critique is his contention that the sort of materialism such functionalism presumes to offer must unwittingly lead to an epiphenomenalism, where physical properties "causally exclude" the mental properties.  I have argued elsewhere (Beakley, forthcoming) that if we adopt any of numerous accounts of causation and 'causal exclusion' from the philosophy of science, then these epiphenomenalist accusations do not bear up under scrutiny. While I stand by that conclusion, here I wish to re-examine Kim's case for functionalist epiphenomenalism in a more oblique way.  My idea is this: instead of grafting on an existing principle of causal exclusion as I did in my previous study, I seek here to ‘rationally reconstruct’ the principle of causal exclusion operative in Kim’s work, from 

the paradigm cases of causal exclusion appealed to in his arguments and the underlying 
methodological constraints alleged to motivate these cases.

That goal is not so far-fetched. Kim himself suggests at one point that his examples and partial explications are sufficient for a working understanding of the principle at work:

The general principle of explanatory exclusion states that two or more complete and independent explanations of the same event or phenomenon cannot coexist.  The meanings of “complete” and “independent” are obviously crucial.  I shall not be offering definitions for these terms; rather I shall focus on some specific cases falling under the intended distinctions, with the hope that, in the course of my discussion, reasonably determinate core meanings will emerge that will give the exclusion principle clear and substantial enough content.  A thorough examination of explanatory exclusion will inevitably spill over into the long-standing debate over the nature of explanation, a topic on which nothing like a consensus now exists.  The discussion to follow will inevitably rest on certain intuitive assumptions about how explanations, especially causal explanations, work; however I hope that the discussion will succeed in showing that whatever model of explanation you accept, unless you take a wholly fictionalist or instrumentalist view of explanation, the principle of explanatory exclusion is a plausible general constraint (Kim 1989a: 250).

As this quote illustrates, two concepts are central throughout Kim’s various statements of his argument: “complete independent cause” (or “complete, independent causal explanation”), and “causal overdetermination”. In fact, together these two concepts 

provide Kim with everything he feels he needs to construct a sense of causal exclusion sufficient for deriving functionalist epiphenomenalism.
This I call “the problem of causal-explanatory exclusion”, for the problem seems to arise from the fact that a cause, or causal explanation, of an event, when it is 
regarded as a full, sufficient cause or explanation, appears to exclude other independent purported causes or causal explanations of it (Kim 1989b:281).

The foregoing I call the problem of “causal–explanatory exclusion”.  For the considerations of the sort we have just seen, when generalized, seem to show that there can be at most one complete and independent causal explanation, or one fully sufficient cause, for any single event.  …Thus the core of the exclusion problem is to answer the question: Given that every physical event has a physical cause, how is a mental cause also possible? (Kim 1991:  291-2).
 
The main motivation Kim provides for this principle of causal exclusion is that, if there is more than one complete independent cause for a given effect, we are (so it is claimed) faced with causal overdetermination. 
If m and p are each countenanced as an independent sufficient cause of b, we would have to conclude that b is overdetermined, which seems absurd (Kim 1991: 291)

In this situation, moreover, the causal role of the pain is in danger of preemption by [supervenience base] N.  This is “the problem of causal/explanatory exclusion”: For any single event, there can be no more than a single sufficient cause, or causal explanation, unless it is a case of causal overdetermination.
(Kim 1996: 150)

Surely this much is intuitively obvious: causal overdetermination is in some sense unacceptable in principle; and causal overdetermination is simply a matter of having more than one complete, independent cause of the same effect.  There follows, then, a principle of causal (-explanatory) exclusion barring such noxious consequences: there can be no more than one complete, independent cause of a given effect.

As I have urged in my earlier study, however, the limpid self-evidence of the epiphenomenalist argument is best appreciated from a distance, through the gauze of intuition and thumbnail judgment; it tends to bear up less well under detailed scrutiny.  At the risk of seeming over-cautious, then, I will unpack these intuitive statements rather more minutely, making clear the precise structure of Kim’s argument.  In so doing, I will  attempt to reconstruct the exact sense of “complete” and “independent” in Kim’s “complete, independent cause” – and, by association, both the precise kind of “causal exclusion” he appeals to, and the sort of “causal overdetermination” he finds so unappealing. Throughout I seek both the minimal requirements needed to fit Kim’s examples of good and bad causal attribution and exclusion (so as not to saddle him with stronger commitments than his examples require), and the maximal charity in avoiding attributions of falsehood or absurdity.  Having isolated what I take to be the most viable reading of these concepts, however, I will suggest that in the end that reading motivates only a weaker principle of causal conclusion than Kim requires to achieve his epiphenomenalist indictment of functionalism.  If my analysis is correct, Kim’s derivation of epiphenomenalism from functionalism systematically equivocates between a weaker principle, for which he can offer evidence, and a much stronger one which he needs to complete his argument.  Kim’s argument is then either invalid, or relies on an extremely strong principle of causal exclusion unmotivated by any of the judgments or methodological principles he offers as evidence.

II. Complete Causes. Turning first to “complete” causes, note that while Kim nowhere defines “complete,” he does, from the frequency of its use, seem to favor one reading in particular: his examples of “complete causes” are most often sufficient conditions.  In one 
passage discussing how two contributing factors C and C*combine to form a larger, and more “complete,” cause, Kim says:
If C and C* are related in this way we do not have two complete explanations – in one sense of “complete explanation”, namely one in which a complete explanation specifies a sufficient set of causal conditions for the explanandum (Kim 1989a: 251).

And elsewhere, in criticizing functionalist mental causation, Kim  considers, then rejects, the possibility that the mental and physical are each such (insufficient) causes combining into a more complete total cause, and proceeds:
Could it be that the mental cause and the physical cause are each an independent sufficient cause of the physical effect?  The suggestion then is that the physical effect is overdetermined.  …This picture is again absurd….   (Kim 1989b: 280-1).

That this second passage runs so parallel to the first – and especially that it brings out the problem of causal overdetermination – suggests strongly that Kim is viewing “complete” causes as sufficient conditions. 

Other passages rely equally unambiguously on the sufficient condition reading of “completeness”.  Kim appeals, for example, to
a broadly nomological conception of causality, roughly in the following sense: an instance of M causes an instance of N just in case there is an appropriate causal law that invokes the instantiation of M as a sufficient condition for the instantiation of N (Kim 1993a: 351).

And more recently, he dispenses with the word “complete,” and states his principle of causal exclusion flatly in terms of sufficient conditions:
This is “the problem of causal/explanatory exclusion”: For any single event, there can be no more than a single sufficient cause, or causal explanation, unless it is a case of causal overdetermination.(Kim 1996: 150)

Yet he does stress, in the first passage, that this is only one sense of the term; and some of his examples do not, at first glance, fit this sense so well.  

I think, at a minimum, that Kim’s tendency to appeal to sufficient conditions stems from his concern to clarify cases of causal overdetermination, in the face of our natural judgments about causation.  Consider again the above sort of case: if there are two “independent” causes which are neither sufficient conditions for the effect, we have an instinctive tendency to say they are two parts of one overall “complete cause” – i.e., two contributing factors  in the overall production of the effect.  In this sort of case, calling each factor “incomplete” means it wasn't itself a sufficient condition;  for when we speak of something being a “(merely) contributing factor” we typically mean that factor was itself insufficient to bring about the effect. (Of course, the factor must still be relevant to the occurence of the event – making it likelier, to put the point in probabilistic terms – on pain of which we would not have said it "contributed" to bringing about the effect.)  My point is simply this: if we generally follow our instinctual tendency to lump together insufficient causes as contributing factors in one unified and more sufficient cause, then it looks like the only discernible cases of causal overdetermination would be those where there are two sufficient conditions.  So Kim’s preference for sufficient conditions in his 

examples may only reflect an attempt to counteract this instinctive tendency, and need not reflect a commitment to a sufficient condition model of causation.  

But what other readings are open to him?  One possibility would be the complete cause as exhaustive set of factors contributing to the occurrence of the effect (where, again, “contributing” means something analogous to “positively relevant”). That would certainly allow the “complete cause” to be a sufficient condition, when the exhaustive set of contributing factors guaranteed the effect; but when the exhaustive set of contributing factors fell short of a sufficient condition, this set would still (being exhaustive) constitute the “complete cause” of the effect.  And then Kim’s use of sufficient conditons in many of his examples would, again, only be a way of overriding our instinct to lump factors together, so that he can bring out the noxious possibility of causal overdetermination he sees in functionalist mental causation.
  
The appeal of this second, “exhaustive set” reading of “complete cause” becomes clearer when Kim describes cases of undeniable causal overdetermination – in his examples, a fire started by faulty wiring and a lightning strike, or a man shot dead by two different assassins, where each cause is sufficient:
C and C* are distinct and each a sufficient cause of E.  We may think of them as belonging to two distinct and independent causal chains.  This then is a case of causal overdetermination (Kim 1989a: 252).

While conceding that "it isn't obvious in cases like these just how we should formulate an explanation of why or how the overdetermined event came about," (ibid.) Kim nonetheless tenders the follow judgment on the situation:
…it is not implausible to think that failing to mention either of the overdetermining causes gives a misleading and incomplete picture of what happened, and that both causes should figure in any complete explanation of the event.  If this is right, the present case is not one in which two complete and independent explanations are possible for one event (Kim 1989a: 252).

Kim notes that the sense of “complete” cause invoked here is different from the previous case where it was explicitly construed as a sufficient condition, but suggests that "this does not preclude a broader sense covering both" (Kim 1989a: 253).  And reading “complete cause” as the exhaustive set of contributing factors provides one such broader sense, since (again) sufficient conditions would just be a special case of such an exhaustive set.  The only peculiarity of the overdetermination cases, on this reading, is that a proper subset of the factors would constitute a sufficient condition.   

Alternatively, Kim suggests, we could simply “exempt all overdeterminative cases from the requirement of explanatory exclusion” (Kim 1989a: 253).  In that case the overdetermination examples would no longer provide a way of deciding which reading of “complete cause” best fits Kim’s claims – though his earlier statement that sufficient 
condition is only one sense of “complete cause” would still provide some motivation for taking the more inclusive “exhaustive set” reading of the phrase.  

On reflection, however, the prospects for the exhaustive set reading of “complete” cause are less than promising.  Note first that, on this reading of “complete,” there can be only one complete cause, by definition.  And then the principle of causal exclusion immediately becomes vacuous – it is itself, we might say, simply (part of) an implicit definition of “complete cause”. But such a vacuous principle of causal exclusion would then be powerless to block mental causes: the interactionist dualist, the occasionalist dualist, the functionalist, and the reductive materialist would all equally accept such a principle without qualms – given that “the exhaustive set of causal factors” is a definite description, it is a matter of logic that there can be only one. And a principle appealing to this sense of “complete” could not rule out supervening mental causes without independently assuming that mental causes do not show up anywhere in the exhaustive set of contributing factors.  Obviously an argument against supervening mental causes that assumes its conclusion as one of its premises exhibits an unwholesome circularity.

It might seem that the missing “independent” half of “complete, independent cause” could still save the day, and spare the exhaustive set reading of the exclusion principle from vacuity.  But any such appearance is an illusion: adding “independent”, however that term is read, cannot help, since adding conditions to a prohibition can only change it for the weaker, if at all.  (Consider: since “No bachelors can be married” is already an empty prohibition, changing it to “No American bachelors can be married” cannot strengthen the constraint.)  So the exhaustive set reading of “complete cause” seems unworkable.

As a variation on the “exhaustive set” reading, consider what we might call the “saturated set” reading of “complete”.  Here, a complete cause is a set of causally relevant factors such that adding any additional factor to the set would ‘make no difference’ to the occurrence of the effect.  On a probabilistic reading of “causally relevant,” for instance, a “saturated set” of factors is a set of factors each positively relevant to the occurrence of the effect, and such that adding any further factor to the set will not increase the conditional probability of the effect.  On a counterfactual reading, the “saturated set” of factors might be a set of factors each counterfactually relevant to the effect (in the sense of being a counterfactually necessary condition for the effect), and such that any further factor would not be counterfactually relevant.  And so on.  

On this reading, causal overdetermination would involve two distinct sets of factors, each of which could not be made more causally relevant to the effect by adding any further factors.  Neither of these sets need constitute a sufficient condition for the effect, so long as they were ‘tied’ for causal relevance, on some reading of “causal relevance”.  And, pending an explication of that phrase, the exclusion principle, barring two or more such complete causes, is not obviously vacuous. 

Nonetheless, I submit that prospects are no brighter for the “saturated set” reading of  “complete cause” than for the “exhaustive set” reading.  Note that in illustrating what a saturated set of factors could be, I was thrown back on different readings of “causally relevant” – the probabilistic reading, the counterfactual reading – because the definition of “saturated set of factors” appeals uncritically to the phrase “causally relevant”.  This is a bad turn, for two (related) reasons.  Most immediately: the reconstructed principle of causal exclusion on this reading, relying as it does on the concept of “complete cause,” must in turn rely on the concept of “causal relevance”.  But the very point of the principle of causal exclusion was to reveal when certain factors are causal irrelevant to the effect – and hence causally excluded by the genuinely relevant factors.  Factors which are causally excluded are just factors causally irrelevant to the effect – so our ‘explication’ of the principle of causal exclusion, on the “saturated set” reading, would be only a circle of synonyms.  The principle of causal exclusion thus requires, as a proper part, a complete principle of causal exclusion.  Put otherwise: adopting this ‘reading’ of “complete cause” is tantamount to leaving open what that term means – and so undercuts any attempt to justify or evaluate Kim’s claims.  In addition, as argued in my previous study (Beakley forthcoming), it is by no means clear that falling back on such going accounts as the probabilistic or counterfactual models of causation provide any support for Kim’s epiphenomenalist argument.

So in terms of specific intepretations, I think we are after all left only with the sufficient condition reading of “complete cause”.   And there is indeed nothing in this definition of “complete cause” that rules out more than one such complete cause.  But then Kim’s position on causal exclusion seems muddled precisely when it comes to the motivating cases of causal overdetermination.  A case where there were two (or more) distinct “complete causes” – i.e. sufficient conditions – is the sort of case which Kim seems to find most troubling for supervenient mental causation; but it is precisely here that his judgments on causal exclusion seem maximally contrary to a case for excluding supervening mental properties. For Kim himself permits overdetermination, in his previous examples of the fire and the murder.  And there he holds out two different options: either read “complete” in a way such that neither cause is complete, or waive causal exclusion for cases of overdetermination.  On the sufficient condition reading of “complete” the first option is not open; but on the second option cases of causal overdetermination are exempted from the causal exclusion principle by fiat, and the 
attempt to causally exclude mental causes – even such as would overdetermine the effect – is abandoned.  

Still, the situation is not beyond hope.  What is needed here is a corollary reconstruction of Kim’s judgments concerning what is, and what is not, an ‘acceptable’ case of causal overdetermination.  In fact, Ned Block has elsewhere suggested a workable reading of the offensive sense of overdetermination which supervenient mental causation threatens. If supervening mental causes overdetermine the effect, Block observes, then every case of mental causation (indeed, of causation by any “second-order property”) would suffer from overdetermination (Block 1990: 158).  By virtue of the general link between mental and physical that supervenience provides, overdetermination would in that case be systematic overdetermination; whereas Kim’s acceptable cases of overdetermination are incidental, only occasional cases of overdetermination (in the sense that, e.g., not every fire or death involves two or more causes) – hence not systematic.  Kim himself remarks at one point on this systematic overdetermination which supervenient mental causation seems to incur: “It is at best extremely odd to think that each and every bit of action we perform is overdetermined in virtue of having two distinct causes” (Kim 1989a: 247).  And interpreting him as objecting only to such systematic overdetermination draws the line between the cases exactly where he wants it.  So I think the sufficient condition reading of “complete cause” is salvagable on this score.  

Finally, note that on the sufficient condition reading, the principle of causal exclusion will only rule out cases of two (or more) sufficient conditions – allowing less-than-sufficient cases of overdetermination to pass unscathed.  The principle of causal exclusion on this reading would then have a somewhat restricted scope – not a critical flaw in itself, but (if we admit breaches of determinism, and “chancy” causation) having the effect of limiting the principle of causal exclusion to only certain cases of causation.  However, since practically every critical example Kim examines involves (competing) sufficient conditions, this point should not affect whether his argument for functionalist epiphenomenalism stands or falls.

III. Independent Causes and “Dependence Relations”. Kim spends less time in his arguments discussing “independence”, and providing distinct examples, than he does with “completeness”; but occasionally he appears to single out “independence” in a fruitful way.  For instance, assuming a principle of causal closure for the physical – where every physical event is caused by a physical event, so that there are no miraculous, non-physical causes of any physical event – Kim phrases his problem with causal overdetermination as follows:
Could it be that the mental cause and the physical cause are each an independent sufficient cause of the physical effect?  The suggestion then is that the physical effect is overdetermined.  So if the physical cause hadn’t occurred, the mental cause by itself would have caused the effect.  This picture is again absurd….  it makes no sense to think that there might be an independent, perhaps telekinetic, causal path from the pain to the limb movement.  Moreover, the overdetermination idea violates the causal closure principle as well…. For the idea that the mental and the physical cause are each an independent sufficient cause involves the acceptance of the counterfactual that if the physical cause had not occurred, the mental cause would have occurred and caused the physical effect.  This is in violation of the causal closure principle (Kim 1989b: 280-1).

Now, for reasons I have set out elsewhere, it is not obvious to me that the counterfactual Kim rejects here really is false, or that the physical cause fares the counterfactual test any better; so I am not convinced Kim’s counterfactual treatment of “independent cause” here is viable (Beakley forthcoming: Section VI).  But the point to focus on is his discussion of “independent” causes: the scenario Kim rejects is one where there is “an independent, perhaps telekinetic, causal path from the pain to the limb movement”.

By way of explicating this concept of “independence,” I take the morphology of the word at face value: two events are independent if each event does not depend on the other.  And what makes that point more than a truism is Kim’s extensive discussion of what he terms “dependence relations,” a discussion spanning several decades of his work.  In an early paper Kim already uses this notion, citing what he takes as its most obvious example: “The causal relation is a paradigmatic case of what I shall call relations of ‘dependency’ or ‘determination’ between events and states”; but he goes on immediately to add that there “appear to be dependency relations between events that are not causal” (Kim 1974: 22).  As examples he cites logico-semantic relations (“Socrates dying” and “Socrates’ wife becoming a widow”) and the part-whole structure of actions and events (Kim 1974: 23-4; 25ff.; 30-1).  Significantly, two decades later he offers the same roster of dependence relations: in explaining the generic covariance of mind and body we must appeal to some or another dependence relation, and this might be causation, mererological supervenience, or some sort of meaning dependence (Kim 1993b: 167).  Likewise, in his classic study on supervenience, Kim begins by noting that – putting logical relations of entailment to one side – we are faced with two fundamental objective dependence relations, viz. causation and mereological supervenience (Kim 1984a: 53-4).  The focal role of this concept, and even his rough taxonomy of types, stretches throughout Kim’s work.

And while he allows that he has not provided an exact definition of the term, Kim does offer at least a partial characterization of what a “dependence relation” amounts to.  Most obviously, Kim insists that “modal force is arguably a necessary aspect of any significant dependency claim” (Kim 1990: 143); and calling this in the same passage “modal or subjunctive force,” Kim clearly has in mind the ability of causal and supervenience claims to license counterfactuals.  

Dependence relations also seem to exhibit certain structural characteristics: we are told, for instance, that “dependence, or determination, is usually understood to be asymmetric” (Kim 1990: 144).  Just as laws will not yield causal relations without introducing temporal anistropy, Kim claims, so the generic covariance of supervenience will not yield a genuine dependence relation without introducing the asymmety of, e.g. mereological supervenience (Kim 1990: 146-7). This is why simple supervenience, defined as modal covariance, is not a genuine dependence relation for Kim, but an umbrella term covering perhaps several such relations (Kim 1993b: 165ff).  In addition, he notes that “supervenience, whether in the sense of covariation or in the sense that includes dependence, is transitive,” and immediately draws a parallel here with causation (Kim 1990: 157).

Finally, note that it is identifying a dependence relation between objects or events that constitutes for Kim the essence of scientific explanation. With mereological supervenience the base event determines the supervenient event, and so in some sense explains that event, just as in causation the cause determines, and hence explains, the effect – the main difference between the two being that causation is a diachronic relation, while supervenience is synchronic:

Mereological supervenience views the world as determined along the part-whole dimension, whereas the causal determinism views it as determined along the temporal dimension (Kim 1984b: 102).

And likewise: 

This Democritean doctrine of mereological supervenience, or microdeterminism, forms the metaphysical backbone of the method of microreduction, somewhat the same way that the principle of causal determinism constitutes the objective basis of the method of causal explanation (Kim 1984b: 96-7)

It is the existence of such dependence relations, as objective relations holding among objects and events in the world, which underwrites what Kim calls his “explanatory realism” – the view that explanations pick out real, explanatory relations among objects in the world: “explanatory or reductive connections, as essentially epistemological 

connections, must themselves be grounded in the objective determinative connections holding for the events in the world” (Kim 1984b:102).

I think the category Kim refers to here as “dependence relations” is very suggestive, and that it can be extended considerably further.  And if we grant Kim that both supervenience and causation are different types of dependence relations, there is a very effective way of spelling out what Kim’s concept of “independent” causes amounts to, and even how two such independent causes would result in genuine causal overdetermination. Quite simply: A and B are independent causes of C exactly if A and B each cause C, while there is no dependence relation between A and B.  

Consider again a familiar case which seems to illustrate the repugnance of causal overdetermination: the barometer example.  We conclude that the barometer example is not, in the end, a case of causal overdetermination of the rain precisely when we find a dependence relation between the drop in barometer needle and drop in atmospheric pressure: the drop in atmospheric pressure caused the drop of the barometer needle (just as it also caused the rain).  If there were no dependence relation between the dropping barometer needle and the dropping atmospheric pressure – so that neither one supervened on the other, and neither one caused the other – then we would have a genuine case of 

causal overdetermination (barring, of course, finding out that both of them were determined – for example, caused – by a yet further factor, which also caused the rain).  

Note how well this proposal fits Kim’s examples of “independent” causal explanations.  He holds, for instance, that if we find that one prima facie cause of the effect supervenes on the other, the appearance of overdetermination vanishes:
C is distinct from C*, but in some clear sense “reducible” to, or “supervenient” on, C*.  …we do not have in cases of this kind two independent causal explanations of the same event.  The two explanations can coexist because one of them is dependent, reductively or by supervenience, on the other (Kim 1989a: 251; Kim’s emphasis).

In this case we find a dependence relation between C and C* (specifically, the supervenience relation), and the competition that would yield causal overdetermination ends; C and C* are no longer viewed as independent causes.  Likewise, if we find that one prima facie cause of the effect is caused by the other, Kim holds that again the appearance of overdetermination vanishes:
C and C* are different links in the same causal chain leading, say, from C to C* and then to E.  In this case again we do not have two independent causal explanations; the explanans of one, C*, is causally dependent on the explanans of the other, C (Kim 1989a: 252; Kim’s emphasis).

Here again the presence of a dependence relation between C and C* (in this case, 

causation) dispels the threatened independence of C and C* – and with it, the threatened causal overdetermination.

In addition, consider how Kim illustrates the risk of causal overdetermination in a case of mental causation: 
If the pain is to have a causal role here, it must somehow ride piggyback on the causal chain from [supervenience base] N to the wincing.  It is implausible in the extreme to think there might be two independent causal paths here, one from N to the wincing, and one from the pain to the wincing (Kim 1996: 150).

Kim insists that the supervening mental cause must bear the dependence relation of supervenience to the physical base cause – barring which we would have two “independent causal paths”.

Finally, this characterization of dependence relations and “independent” properties makes 
clear the difference between a pair of properties being “distinct,” and their being “independent”:
Granted that [mental property] M1 is supervenient on, and dependent on, [physical base] P1, and hence not an independent cause of P2; so long as M1 remains a distinct property not identified with P1, we must, it would seem, still contend with the two purported causes of a single event (Kim 1993c: 361).

As this passage illustrates clearly, “independence” as Kim uses the term is not synonymous with the “distinctness” that comes, e.g., of non-reductive properties: while any pair of independent properties (events, etc.) must be distinct, two distinct (i.e., non-identical) properties could still bear a dependence relation between them.

We are left with the following definition of when two events are “independent”:

X and Y are independent iff no dependence relation holds between X and Y.

And then the definition of “independent cause” follows as a particular case:

X and Y are independent causes of Z iff both X and Y bear the causation relation to Z, and no dependence relation holds between X and Y.

We could of course construct a parallel definition of “independent supervenience base”.  

IV. Complete Independent Causes and the Principle of Causal Exclusion.  So far independence proves more revealing for causal exclusion than did completeness. If the principle of causal-explanatory exclusion is intended to shield us from cases of causal overdetermination, such a principle needs, at a minimum, to block cases of multiple independent causes of the same effect.  Let us set down this minimal requirement:
Weak Principle of Causal Exclusion, “Independence Version” (Weak PCE-I): Two distinct events, X and Y, cannot be independent events that both bear the causal dependence relation to Z (where, as defined above X and Y are independent events iff there is no dependence relation between them).  More briefly: X and Y cannot be independent causes of Z.

But now I am neglecting “completeness” for “independence,” whereas Kim includes both in his principle of causal exclusion.  What difference is made by adding “completeness” to the Weak PCE-I?

Since, as already noted, additional conditions cannot strengthen the exclusion rule, the question is not whether Weak PCE-I is too weak – adding “completeness,” or any other condition, cannot change that – but whether it is too strong.  And on reflection, that is certainly the case.  Without specifying which sort of factor(s) must be independent, the Weak PCE-I would run amok, inappropriately excluding even the proper parts of a sufficient cause, i.e., each individual contributing factor.  Here’s how: suppose there is only a single, sufficient condition C that causes effect E (so no troublesome overdetermination in this case); and suppose C is itself composed of proper parts, or contributing factors, P1, P2, and P3, each necessary conditions for E, and each independent of the others (in our sense of “independent”).  As it stands, the Weak PCE-I would allow (say) P1 to causally exclude P2 from E, in the following manner: if we say that even an incomplete cause bears the causal “dependence relation” to E, then both P1 and P2 bear this relation to E – precisely the sort of case prohibited by the Weak PCE-I.  But then the Weak PCE-I would bar even a case of a single sufficient condition for an effect.  So we need to specify that not just any causal factor is grist for the Weak PCE-I; and adding “complete” to this principle, on the sufficient condition reading, has precisely this effect.

(Another way of putting this point is to note that, as part of my troublesome counterexample, I viewed necessary-but-insufficient conditions as each bearing the causal dependence relation to the effect E.  Perhaps that is illicit – perhaps we should not allow necessary-but-insufficient conditions to count as cases of causation.  The discussion must then turn again to the question of which model of causation we allow – the very discussion this paper seeks to steer clear of, by reconstructing the principle of causal exclusion independently of any specific model of causation.  And we avoided such a discussion earlier precisely by restricting our focus – with Kim – to sufficient causes.   By adding “complete” to the Weak PCE-I – on our reading of “complete” causes as sufficient causes – we do precisely that.

On the "sufficient condition" reading of “complete”, the principle of causal exclusion bars two or more sufficient conditions for the same effect which are independent – i.e., where neither causes the other, nor supervenes on each other.  So, adding “complete” to our principle as required, we get:  

Weak Principle of Causal Exclusion (Weak PCE): Two distinct sets of properties, X and Y, cannot be complete independent causes of Z.

This is, of course, precisely Kim’s wording of his principle of causal exclusion: “The general principle of explanatory exclusion states that two or more complete and independent explanations of the same event or phenomenon cannot coexist” (Kim 1989a: 250). And since it agrees not just with his wording, but with all of the motivating examples surveyed above, I submit that this constitutes the minimal requirement on causal exclusion. 

V. Strong and Weak Principles of Causal Exclusion.  All of this, of course, was preliminary to an evaluation of Kim’s case for functionalist epiphenomenalism.  So we turn back to his argument at last.

And here we note something of critical importance: in arguing that functionalism entails epiphenomenalism, I submit, Kim must employ a much stronger principle than the Weak PCE – and hence a much stronger principle than is required simply to block systematic causal overdetermination, and fit the cases he offers as evidence.  For in the case where C supervenes upon C*, Kim agrees that we do not have two independent causes of the effect, E – and for precisely that reason, the threat of causal overdetermination is averted.  That is: since functionalist mental states are taken to supervene upon the physical, they will not qualify as independent causes – and so they will not violate the Weak PCE.  To make a case that the supervenient mental is epiphenomenal, Kim must say more than just that a single effect cannot have two independent causes – that much is demanded by the Weak PCE, but (given our definition of “independent”) that much is a condition already obeyed by supervening properties.  To reach his functionalist 
epiphenomenalist conclusion, Kim must go on to hold that a single effect cannot have two causes at all (whether those causes are independent or not).

Let us call that principle the Strong Principle of Causal Exclusion:

Strong Principle of Causal Exclusion (Strong PCE): Two distinct sets of properties, X and Y, cannot both be (complete) causes of Z.

And now we see the pivotal question for Kim’s argument: what motivation is provided, by any of the examples Kim offers, to adopt the Strong PCE over the Weak PCE?  Unacceptable cases of systematic causal overdetermination seemed to be Kim’s primary motivation in arguing for a principle of causal exclusion; yet the Weak PCE blocks those just fine. Since supervening mental properties are not independent of their physical bases, they will not in any case constitute a second independent cause – and, as we have seen from various passages in Kim’s own words, the type of overdetermination he finds so troubling is the case where a single event has two complete, independent causes.  The cases which Kim feels do not qualify as offensive overdetermination (two sufficient causes of a fire or a death) are properly permitted by the Weak PCE, so long as we recognize that overdetermination becomes noxious only to the extent that it is systematic. 

But if there is no further reason to prefer the Strong PCE over the Weak, then Kim’s preference for the Strong PCE seems like a setup – that is, it seems that the sole purpose in preferring the Strong PCE over the Weak PCE is to argue for functionalist epiphenomenalism.  Now, if we need an argument to convince us that supervenient mental causation is ruled out by causal exclusion, then surely supervenient mental causation is to that degree a controversial case.  But Kim means to motivate such an exclusion principle from uncontroversial, agreed-upon cases, which could then settle the controversial question of mental causation.  So of course a preference against supervenient mental causation cannot, in the present dispute, itself be a reason for tipping the scales in favor of the Strong PCE – it being precisely the case at issue.  If the Weak and Strong PCE both save all the undisputed phenomena, then Kim’s preference for the Strong PCE, in his argument for functionalist epiphenomenalism, would be flatly circular.

VI. Further Evidence.  So we ask: have we overlooked some bit of motivating evidence in our rational reconstruction of Kim’s argument? Are there further uncontroversial cases which our principle of causal exclusion must also fit? 

Kim does sometimes vary his presentation of his basic argument; and in so doing, he appeals on occasion to three additional principles: (1) the principle of the causal closure of the physical; (2) the principle of the causal discernment of natural kinds; and (3) the principle of simplicity/unification.  All of these, as Kim recognizes, are methodological principles, purported to hold for scientific theories and inquiry.  I examine them in turn.  

1. Causal Closure of the Physical. In accusing supervenient mental causation of engendering an offensive causal overdetermination, Kim says this is not only offensive in itself, but a violation of an additional principle: the causal closure of the physical domain.
There is a further assumption that I believe any physicalist would grant, namely “the causal closure of the physical domain”.  Roughly, it says this: any physical event that has a cause at time t has a physical cause at time t.  This is the assumption that if we trace the causal ancestry of a physical event, we need never go outside the physical domain (Kim 1989b: 280).

In the terminology I am using here, this principle says: every physical event has a (complete and independent) physical cause, if it has any (complete, independent) cause at all.  Consistent with this principle, a physical event might have no physical cause at all – but then it had better not have a (complete, independent) mental cause instead (or any other sort of non-physical cause).  

Yet the functionalism in question here is, as Kim recognizes, a form of non-reductive materialism, wherein the mental everywhere supervenes upon the physical.  So violation of “the causal closure of the physical” by functionalist mental states is already blocked: any time there is a mental cause of a physical event, the mental cause, however complete, will (by virtue of supervenience) not be an independent cause of the physical event – so there will (by virtue of materialism) always be a physical cause on which the mental cause supervenes. So Kim’s principle of the “causal closure of the physical” is already preserved by the Weak PCE.

My point here turns around the word “need,” in Kim’s claim that “if we trace the causal ancestry of a physical event, we need never go outside the physical domain”.  On a physicalist functionalist view employing the Weak PCE, one indeed need never go outside the physical in explaining the physical; but that alone does not imply that one may not go outside the physical – and only the latter is required for functionalist mental causation.  That is, saying we need not go outside the physical to explain the physical says only that it is not necessary (neither Weak nor Strong PCE would say that on a physicalist model); whereas saying that we cannot do so, says that it is not possible (only the Strong PCE says that on a physicalist model).

And this more moderate reading of “causal closure” is supported by Kim’s other statements of the principle:
…suppose then that we have somehow put together an account of how mental events can be causes of physical events.  But these events, qua physical events, must have physical causes; it surely would be an anachronistic retrogression to Cartesian interactionism to think that there are physical events that have only nonphysical causes.  To countenance such events would amount to the rejection of the causal-explanatory closure of the physical domain, a principle that seems minimally required of any serious form of physicalism  (Kim 1991: 290).

Here again, the causal closure of the physical is violated precisely when a physical event has only a non-physical cause; but here again, a physicalist functionalism, following the Weak PCE, will then obey the causal closure of the physical.

Kim does later offer a different statement, of what he calls “the principle of physical causal closure”: “If you pick any physical event and trace out its causal ancestry or posterity, that will never take you outside the physical domain” (Kim 1998: 40).  This principle seems to say that the causal ancestry of each physical event must consist entirely of physical events.  If that reading is correct, this principle is much stronger than the closure principle Kim used previously (quoted above) – indeed, it amounts to a stipulation that non-physical properties are epiphenomenal; and of course, if functionalist epiphenomenalism is controversial enough to merit arguments, then a mere stipulation should not in itself be compelling.

But anyhow Kim’s phrasing of this latter principle is, perhaps, simply a bit imprecise.  For notice that Kim immediately goes on to claim: “If you reject this principle… you would be saying that any complete explanatory theory of the physical domain must invoke nonphysical causal agents” (Kim 1998: 40, my emphasis).  Now, if Kim intended the stronger reading of the principle, that every physical event must be explained solely by physical causes, then its negation (its “rejection”) would be that we need not explain every physical event solely by physical causes – far short of the consequence Kim sees here in the rejection of the principle.  On the other hand, if Kim intended only the weaker reading he offered previously – that every physical event can be explained solely by physical causes – then its negation would indeed yield exactly the dire outcome Kim foresees. For the negation of this weaker reading would be the claim that we cannot explain each physical event solely by physical causes – equivalently, that we “must invoke nonphysical causal agents”.  

So: in order for Kim to validly deduce this noxious consequence from the negation of the Causal Closure principle, he must use the weaker version of the principle – and that, as we saw, is perfectly compatible with the Weak PCE, and hence no evidence in favor of the Strong PCE.  Whereas, if Kim does indeed intend the stronger reading, he cannot validly deduce the noxious consequence from its negation, only invalidly, by way of a scopal error involving the interaction of negation and the obligation operator; and short of deriving such an unhappy consequence from the negation of the stronger principle, Kim provides us with no evidence to motivate it (beyond what the weaker principle already handled).

2. Causal Individuation of Properties.  As something of an independent case for his epiphenomenalist indictment, Kim also appeals to what he terms the “causal powers” of properties.  By way of some reasonable claims concerning these “causal powers,” Kim holds, we can see rather easily why supervening mental properties are rendered epiphenomenal – and, as a bit of a bonus, how this constitutes a case for their outright elimination.  Let us consider these principles in turn.

Kim’s first requirement is what he terms “Alexander’s Dictum” (after Lloyd Alexander, to who Kim credits the principle):
This we may call "Alexander's dictum": To be real is to have causal powers.
(Kim 1993a: p. 348)

I am supposing that a nonreductive physicalist is a mental realist, and that to be a mental realist, your mental properties must be causal properties – properties in virtue of which an event enters into causal relations it would otherwise not have entered into. (Kim 1989b: 279)

It is worth noting that, while Kim is nowhere very explicit about what the “causal powers” of a property amount to, in the second quote they involve at least the property’s being a counterfactual-supporting necessary condition for an effect, in some context.  A property completely devoid of “causal powers” would then be a property such that, had it (counterfactually) not been present, every ‘effect’ (subsequent event) would have been exactly the same.  

Alexander’s Dictum immediately has the effect of ruling out any properties which have no “causal powers” – that is, rendering methodologically illegitimate the claim that such exist.  So if Kim can succeed in demonstrating that supervenient mental properties are epiphenomenal, he can turn this to an argument for their outright elimination.  

To this Kim adds a second requirement, which he terms the “Principle of Causal Individuation of Kinds”:
[Principle of Causal Individuation of Kinds] Kinds in science are individuation on the basis of causal powers; that is, objects and events fall under a kind, or share in a property, insofar as they have similar causal powers.  (Kim 1992: 326)

The word “similar” in Kim’s phrasing leaves this principle somewhat ambiguous; but, as becomes evident in further quotes, he takes this principle as a rule for discerning properties (“kinds”), such that causally indiscernible properties will count as identical.  This principle is likewise a methodological rather than a metaphysical principle, since Kim does not (perhaps could not) give us any reason to suppose that distinct properties could not be causally indiscernible; but as a methodological principle, it holds (like Alexander’s Dictum) that we should postulate distinct properties only in order to causally account for some phenomena. 

As a bit of extra support for his Principle of Causal Individuation of Kinds, Kim notes that even Jerry Fodor, in full functionalist mode – and indeed, most philosophers of mind involved in current debates over mental content – endorse the view that scientific properties are individuated in terms of their ‘causal powers’: 
Fodor gives it an explicit statement in Psychosemantics (Cambridge: MIT Press, 1988), chap. 2.  A principle like this is often invoked in the current externalism/internalism debate about content; most principle participants in this debate seem to accept it (Kim 1992: 326, n. 33)

Third, Kim introduces a “Principle of Causal Inheritance”:
[The Causal Inheritance Principle] If mental property M is realized in a system at t in virtue of physical realization base P, the causal powers of this instance of M are identical with the causal powers of P. (Kim 1992: 326)

More recently, Kim phrases the point somewhat differently:
...I find the following principle highly plausible (I have elsewhere called it “the causal inheritance principle”):

If a second-order property F is realized on a given occasion by a first-order property H (that is, if F is instantiated on a given occasion in virtue of the fact that one of its realizers, H, is instantiated on that occasion), then the causal powers of this particular instance of F are identical with (or are a subset of) the causal powers of H (or of this instance of H).  (Kim 1998: 54)

Moreover, Kim is ready with an explanation as to why we should find this third principle credible:
...to deny it would be to accept emergent causal powers: causal powers that magically emerge at a higher level and of which there is no accounting in terms of lower-level properties and their causal powers and nomic connections.  This leads to the notorious problem of "downward causation" and the attendant violation of the causal closure of the physical domain.  (Kim 1992: 326-7)

And, it is worth noting, this point carries over even on the weaker reading of the Causal Closure of the Physical, discussed above.

Bringing these three principles to bear on physicalist (but non-reductivist) functionalism, Kim draws his conclusion :
It is clear that the Causal Inheritance Principle, in conjunction with the Physical Realization Thesis, has the consequence that mental kinds cannot satisfy the Causal Individuation Principle, and this effectively rules out mental kinds as scientific kinds.  The reasoning is simple: instances of M that are realized by the same physical base must be grouped under one kind, since ex hypothesi the physical base is a causal kind; and instances of M with different realization bases must be grouped under distinct kinds, since, again ex hypothesi, these realization bases are distinct as causal kinds.  Given that mental kinds are realized by diverse physical causal kinds, therefore, it follows that mental kinds are not causal kinds, and hence are disqualified as proper scientific kinds. (Kim 1992: 327)

So mental properties are (presumably) epiphenomenal, and are thus proven to have no place in a scientific worldview – functionalist epiphenomenalism with a bonus.

Does the argument work?  Let us consider it in somewhat more detail.

By way of concrete illustration, consider an example of mental causation, on the physicalist functionalist scheme.  We have (instance(s) of) a mental property A, which purportedly causes some effect – say, (instance(s) of) some behavioral property E.  And now let us add the requisite premises, one by one, to see what can or cannot happen in such a scenario.  

1.  Since we are considering physicalist functionalism, every A(-instance) must supervene upon (instance(s) of) some physical property B.

2.  Since the functional property A is taken to be multiply instantiable, let us add that each A(-instance) supervenes, in different cases, on (instance(s) of) distinct physical properties B1, ..., Bn (n > 1).

3.  Since we are considering non-reductive materialist functionalism, we add that mental property A is distinct from (i.e., ≠) all the physical properties B1, ..., Bn.

Now we add Kim’s additional principles.

4.  We add Alexander’s Dictum, whereby every property must have some “causal powers” to qualify as real.  So: the properties A, B1, ..., Bn all have some “causal power” (i.e., are each ‘causally responsible’ for some effect(s), in the sense that the effect would not have occurred had that property[-instance] not been present then).

5.  We add the Principle of Causal Individuation of Kinds, whereby no two distinct properties can have exactly the same “causal powers” (i.e., be causally [i.e., counterfactually] responsible for exactly the same effects).  So: among the properties A, B1, ..., Bn, each property causes some effect(s), but no two of these properties will cause exactly the same set of effects.  This establishes that each of the base properties are causally discernable from the rest, and that A must be causally discernable from each  base property Bi.  Concerning this second point, there are three different ways for A to be causally discernible from each base properties, Bi: (i) A causes some effects that Bi doesn’t; (ii) Bi causes some effect that A doesn’t; or (iii) some combination of (i) and (ii).  

6.  We add the Causal Closure of the Physical, whereby every effect that has a cause, must have a physical cause.  Returning to our three options from the last paragraph, we see that this principle rules out option (i): in no instance can A have effects that its physical supervenience base, Bi, does not.  And that rules out (iii) by association.  So we now have: for each base property Bi, Bi causes some effect that A doesn’t – the “causal powers” of the functional property, A, are always a proper subset of the physical base property Bi.  This is, at last, a significant entailment – but still one falling short of functionalist epiphenomenalism, since nothing so far implies that A has no effects whatsoever.  

7.  We add the Principle of Causal Inheritance, whereby the supervening functional property ‘inherits’ the causal powers of its supervenience base properties.  It might appear that we are at this point faced with a genuine reductio: we have established that in each instance A must have a proper subset of the “causal powers” of its supervenience base Bi; but Causal Inheritance seems to suggest that A must have exactly the same “causal powers” as its supervenience base.  Does Kim finally have enough premises in place to refute the functionalist’s claims to non-reducing properties (never mind mental causation)?

In fact, there are two different ways of validly deriving a contradiction – and (at least) two ways of not deriving it.  Whether we reach Kim’s conclusion depends – unsurprisingly – on whether we choose a weaker, or a stronger, version of some key principles.  And I will endeavor to show that Kim – characteristically – provides grounds for believing only the weaker of these, while needing the stronger to derive his conclusion.

While I passed over the point earlier, in the interest of setting out the structure of this argument, the fact is Kim phrases his Principle of Causal Inheritance in two different – and non-equivalent – ways.  The first way:
[The Causal Inheritance Principle] If mental property M is realized in a system at t in virtue of physical realization base P, the causal powers of this instance of M are identical with the causal powers of P. (Kim 1992: 326)

And the second way:
...I find the following principle highly plausible (I have elsewhere called it “the causal inheritance principle”):

If a second-order property F is realized on a given occasion by a first-order property H (that is, if F is instantiated on a given occasion in virtue of the fact that one of its realizers, H, is instantiated on that occasion), then the causal powers of this particular instance of F are identical with (or are a subset of) the causal powers of H (or of this instance of H).  (Kim 1998: 54)

Note that the first phrasing of this principle has the causal powers of A as identical with those of its supervenience base, Bi.  In that case, non-reductive properties of themselves issue in a contradiction in the final step of our argument, and so presumably are ruled out in principle.  But on the second statement, where the functional property need only inherit a subset of the “causal powers” of its base, we are faced with nothing that we hadn’t already derived in Steps 1 through 7.  Since the first statement is more restrictive (ruling out the possibility of inheriting only a subset), I will call it the “Strong Principle of Causal Inheritance”; and the second will then be the “Weak Principle of Causal Inheritance”.  Then comes the now-familiar question: which version of this principle does the evidence support?

Recall that Kim’s motivation for the Causal Inheritance Principle was defense of another, presumably more basic principle, the Causal Closure of the Physical
...to deny it would be to accept emergent causal powers: causal powers that magically emerge at a higher level and of which there is no accounting in terms of lower-level properties and their causal powers and nomic connections.  This leads to the notorious problem of "downward causation" and the attendant violation of the causal closure of the physical domain.(Kim 1992: 326-7)

But neither of the Causal Inheritance Principles allow the functional property to have new “causal powers” – which is to say, the Weak Principle of Causal Inheritance already blocks the “new causal powers” of the functional perfectly well, and we are left with no evidence or motivation favoring the Strong version of this principle over the Weaker.  Once again, Kim needs the stronger of the two principles to derive his conclusion, but he has successfully argued only for the weaker.

Another of Kim’s principles is open to interpretation as well, however.  In the statement of Alexander’s Dictum that we considered above, Kim only has it requiring that a property have some “causal powers” to qualify as real:
This we may call "Alexander's dictum": To be real is to have causal powers.
(Kim 1993a: 348)

I am supposing that a nonreductive physicalist is a mental realist, and that to be a mental realist, your mental properties must be causal properties – properties in virtue of which an event enters into causal relations it would otherwise not have entered into. (Kim 1989b: 279)

But elsewhere Kim gives this principle a considerably stronger statement:
...as we have observed, the emergentist and the nonreductive physicalist are mental realists, and Mental Realism, via Alexander's dictum, entail causal powers for mental properties.  ...Now, mental properties, on both positions, are irreducible net additions to the world.  And this must mean, on Alexander's dictum, that mental properties bring with them new causal powers, powers that no underlying physical-biological properties can deliver. For unless mentality made causal contributions that are genuinely novel, the claim that it is a distinct and irreducible phenomenon over and beyond physical-biological phenomena would be hollow and empty.  (Kim 1993a: 350)

Here, Kim demands that each functional property have some “causal power” over and above those of its supervenience base – a requirement almost custom-built to clash with the Causal Closure of the Physical, by granting the functional new “emergent” “causal powers” beyond those of the physical.  Call this second, more demanding statement “Strong Alexander’s Dictum,” and the earlier, less demanding form “Weak Alexander’s Dictum”.  We are faced once again with the question: which should we believe?  Postulation of epiphenomenal properties – properties with no “causal powers” – is blocked already by the Weak Alexander’s Dictum.  If there is no additional evidence motivating the Strong Alexander’s Dictum over the Weak, then Kim has once again fallen short of his epiphenomenalist endictment of functionalism.

Note also that Kim’s appeal to Fodor as a fellow supporter for his Principle of Causal Individuation of Kinds does nothing to tip the scales in favor of the stronger reading(s) Kim needs.  While Fodor does, in the text Kim cites, endorse the Causal Discernment of Kinds, he does so only to the extent of holding two properties with identical causal powers to be themselves the same property.
...what you need in order to do science is a taxonomic apparatus that distinguishes between things insofar as they have different causal properties, and that groups them together insofar as they have the same causal properties (Fodor 1987: 34)

But this much is perfectly compatible with supervening mental properties which bring no new causal powers to bear – just so long as they are not devoid of causal powers altogether.  Nowhere does Fodor demand that the mental property have some effects that its physical base does not – only that the mental and physical properties not have exactly the same “causal powers”.  And that is achieved so long as the mental properties have some causal powers, but only a subset of the causal powers of its physical base.  So Fodor’s principle of causal taxonomy demands no more than is given by the Weak Principle of Causal Inheritance, and the Weak Alexander’s Dictum.  Once again, the evidence Kim provides is saved by the weaker principles – but he needs the stronger to derive functionalist epiphenomenalism.

Finally, I think it fairly clear the kind of reason Kim has in mind for demanding novel “causal powers” from each property.  It is effectively the motivation he provides for all his causal principles surveyed in this section: Kim is adopting a picture of scientific methodology wherein each entity (specifically, each property) is postulated for the sole purpose of causally explaining otherwise inexplicable phenomena.  That is: I take Kim to hold that there is no other reason for introducing properties into a scientific theory, except for purposes of novel causal explanation.  Added to this is a basic principle of Simplicity, or Parsimony, already at work even in the Weak Alexander’s Dictum, and the Principle of Causal Individuation, both of which are aimed at blocking the ‘extra baggage’ of properties which do not causal explain anything not already causally explained.  On this picture, after the physical properties have causally explained as much of the phenomena as they can, there will (thanks to the Weak Principle of Causal Inheritance) be no causal explaining, of as-yet unexplained phenomena, left for supervening functional properties.

This suggestion perhaps goes beyond “reconstruction,” and shades into speculation about Kim’s guiding worldview.  Nonetheless, beyond some account such as this, Kim suggests no reason to prefer the Strong Alexander’s Dictum to the Weak.  So by way of addressing this possible motivation for the stronger of these two principles, I want to consider the reasonableness of such a view.

In fact, I believe functionalist philosophers of mind (among others) have provided a perfectly sound and straightforward methodological incentive for introducing functional properties that may well causally explain only phenomena which are each also causally explicable in purely physical terms.  Quite simply, the functional explanation offers a simple, unified account of phenomena whose physical explanations would be far less simple and far more disunified.  Simplicity and unification of a broad range of phenomena has, after all, been the main defense provided for functional theory throughout its career.  Hilary Putnam, in a very influential early statement of functionalism, argues (for example, in his peg-and-board analogy) that higher-level 

accounts can provide more explanatory understanding of our domain of study, and one that spans various lower-level implementations.
The same explanation will go in any world (whatever the microstructure) in which those higher level structural features are present.  …Explanation is superior not just subjectively, but methodologically, in terms of facilitating the aims of scientific inquiry, if it brings out relevant laws.  An explanation is superior if it is more general (Putnam 1973: 94-95)  

Fodor, in his classic paper on “special sciences,” likewise presents unification of otherwise disparate phenomena as precisely the reason for taking functionalist properties and theories seriously:
I am suggesting, roughly, that there are special sciences not because of the nature of our epistemic relation to the world, but because of the way the world is put together: not all the kinds (not all the classes of things and events about which there are important, counterfactual supporting generalizations to make) are, or correspond to, physical kinds.  …Physics develops the taxonomy of its subject matter which best suits its purposes….  But this is not the only taxonomy which may be required if the purposes of science in general are to be served: e.g., if we are to state such true, counterfactually supporting generalizations as there are to state.  So there are special sciences, with their specialized taxonomies, in the business of stating some of these generalizations (Fodor 1974: 144-145).

And more recently, in discussion of none other than functionalist epiphenomenalism, Frank Jackson and Philip Pettit reiterate generality and unification as the driving factor behind adoption of functionalist theories: 
We propose in place of an ‘identity theory’, that the causal relevance of dispositions can be captured in terms of what might be called invariance of effect under variation of realization.  …We are often interested not merely in how something in fact came about but also in how it would have come about.  That is why, paradoxically, we can sometimes improve an explanation by, in a sense, saying less.  …A certain piece of behavior will have a certain property, say that of being in the direction of a certain cup of coffee, as a result of the concatenation of very many neurophysiological states which will have given rise to that piece of behavior by virtue of their natures, that is, by virtue of the neurophysiological properties they instantiate.  But, of course, there will be other ways that behavior with the property of being towards the coffee could have been caused, other neurophysiological ways, or even, other non-neurophysiological ways  if we allow ourselves Martian speculations.  …it may be that many of these ways, including the actual way, are united by the functional properties they realize…. (Jackson and Pettit 1990: 205-206)

So I believe there are clear methodological reasons for adopting functional properties in science, regardless of whether they can causally explain a single phenomenon which is causally inexplicable for physical theory.  Insisting as he does on realist, objective readings of explanations (and, presumably, the theories such explanations employ), Kim might be expected to balk at allowing simplicity and unification to shape our choice of ontology.  But I see no escape from this: the realist philosopher (and scientist) follows these constraints just as much as the instrumentalist.  Moreover, the functionalist who proposes parsimony and unification as methodological motivation for postulating functional properties and objects can do so every bit from a realist point of view.  In the above quote, for example, Fodor offers the objects of his higher-level, functional “special sciences” as real properties, not convenient fictions.

And, of course, Kim would be especially ill-positioned to object to such an appeal to simplicity and unification, for the simple reason that he himself appeals to simplicity and unification as possible further evidence for the causal exclusion of supervening functional states.

3. Simplicity and Unification. Finally, Kim sees a link between causal-explanatory exclusion and a general principle of simplicity.  So, for example, when considering a physical effect P*, physical cause P, and mental cause M, he offers simplicity as one reason to dispense with the mental cause: 
First, there is the good old principle of simplicity; we can make do with P as P*'s cause, so why bother with M?  (Kim 1993a: 354)

And likewise:
The exclusion principle… says that for any event more than one complete explanation is excess baggage.  …We can indeed think of the explanatory exclusion principle as falling under the general simplicity requirement…. (Kim 1989a: 260).

Now, the main obstacle to an appeal to such a simplicity requirement will be articulating the concept of “simplicity” in any fruitful degree of detail. In the philosophy of science it is something of a truism that, while the concept seems intuitive enough at first blush, it is of no help as a methodological criterion until we possess an articulated metric for comparing theories for relative simplicity.  In fact, Kim himself notes that the concept is unhelpfully vague: to the suggestion that the principle of simplicity itself could do the work of his principle of causal exclusion, Kim responds that, in the absence of any specific standards of simplicity, a principle of simplicity cannot motivate or justify any further principles.
...it might be said that there is no need to appeal to any special, and potentially controversial, epistemological or metaphysical views concerning explanation in order to justify something like the rule of explanatory exclusion, and that all we really need is Ockham’s razor, the familiar principle of simplicity, that enjoins us to get by with fewest possible entities, hypotheses, theroetical principles, and, of course, explanations.  In reply, I would first note that the general simplicity requirement is vague and its application requires a more precise interpretation of the situation to which it is to be applied.  In particular, we need to determine exactly at what point the entities in question begin to be multiplied “beyond necessity” (Kim 1989a: 259-260).

So long as the principle of simplicity remains non-specific and unclear, then, it can offer no clear or specific support for Kim’s Strong PCE.

But this problem, while critical, is in a sense beside the point.  For, continuing on this last passage, Kim argues that, far from supporting his principle of causal exclusion, the simplicity requirement is itself being supported by the principle of causal exclusion:
In fact, determining where the excess baggage starts is the difficult part; the rest is trivial.  The exclusion principle does the difficult work: it says that for any event more than one complete explanation is excess baggage.  ...It is not at all obvious that considerations of parsimony alone should mandate us to reject all but one explanation.  We can indeed think of explanatory exclusion as a special case falling under the general simplicity requirement: it is a specific rule concerning one important way in which simplicity is to be gained in explanatory matters, and it explains why this form of simplicty is to be desired.  That is, the explanatory exclusion principle provides a rationale for the application of Ockham’s Razor to multiple explanations of a single explanadum (Kim 1989a: 260).

A principle of causal exclusion cannot, then, be supported by the simplicity requirement, since, prior to articulation, simplicity by itself cannot motivate any further principles (such articulation being “the difficult work”).  So, if anything, the principle of causal exclusion is justifying application of  a simplicity requirement to cases of causation and explanation: it is in Kim’s judgment only by way of an independently developed articulation of simplicity – viz., the principle of causal exclusion – that we can even apply the simplicity requirement to these cases.

Despite Kim’s occasional mention of a simplicity requirement, then, he does not appear to take such a requirement by itself to provide any independent motivation for any principle of causal exclusion – and, specifically, none for the Strong PCE.

So taking even these three further factors into account, I conclude that every example Kim assigns to a principle of causal exclusion can be handled perfectly well by the Weak PCE. And this is what makes Kim’s argument so interesting: he appears to agree with every premise I am offering in defense of functionalist mental causation; but then at the last step, he claims he has derived epiphenomenalism.  He sums up his case for functionalist epiphenomenalism very succinctly:
In any case, physicalism must respect the basicness and priority of the physical, and this must include respect for the basicness of physical causation.  If physical facts determine all the facts, it must be the case that physical facts, including causal facts about physical events and states, must determine all the facts about mental causation.  For this reason, any “physicalistically correct” account of mental causation must inevitably make mental causation dependent on, and derivative from, physical causation, thereby exposing itself to the charge of epiphenomenalism (Kim 1993c: 360).

My account agrees with everything Kim says here, up to – but definitely not including – the last eight words.  To derive the conclusion validly, Kim must invoke the Strong PCE.  Yet, I have argued, the Weak PCE gives the right results in all the uncontroversial cases; and with the Weak PCE alone, Kim’s epiphenomenalist conclusion does not follow from his (quite acceptable) premises.  Once again, the causal power of functional states is of course the point in dispute, so it cannot itself be called on to lend preference to the Strong PCE except in a circular way.  So, once again, the argument for functionalist epiphenomenalism does not stand.

The same point is evident in the following passage:
The foregoing I call the problem of “causal–explanatory exclusion”.  For the considerations of the sort we have just seen, when generalized, seem to show that there can be at most one complete and independent causal explanation, or one fully sufficient cause, for any single event.  …Thus the core of the exclusion problem is to answer the question: Given that every physical event has a physical cause, how is a mental cause also possible? (Kim 1991: 291-2).  

I take it the closing question is rhetorical: in light of the earlier sentences in this passage, it is taken to be clear that a mental cause is not also possible.  But note, once again, that this negative answer to the italicized question does not follow from what came before.  The principle of causal exclusion here states that “there can be at most one complete and independent causal explanation” – this is just the Weak PCE.  But given the Weak PCE, and assuming in addition that “every physical event has a physical cause,” the functionalist has no problem answering the question “how is a mental cause also possible?”  It is only assuming the Strong PCE – that every event (and so, specifically, every physical event) has only one complete cause, simpliciter – that we face a problem answering this question.

Note further that in this last passage Kim gives our statement of the Weak PCE – and then immediately offers an alternative phrasing: “there can be at most one complete and independent causal explanation, or one fully sufficient cause, for any single event.”  In light of our discussion so far, the second statement is hardly an innocent paraphrase of the first.  On the contrary, the second phrasing, by leaving out the word “independent” (or any synonym or cognate), just is the Strong PCE.  Nor is this the only case where Kim lapses without warning into the Strong PCE:
The exclusion principle does the difficult work: it says that for any event more than one complete explanation is excess baggage (Kim 1989a: 260).

And likewise, 
In this situation, moreover, the causal role of the pain is in danger of preemption by [supervenience base] N.  This is “the problem of causal/explanatory exclusion”: For any single event, there can be no more than a single sufficient cause, or causal explanation, unless it is a case of causal overdetermination.  (Kim 1996: 150)

In light of his typical inclusion of “independence” in his principle of causal exclusion, the variation in this last quote cannot appear innocent (or even consistent).  As there is indeed a difference between which principle of causal exclusion we get with “independence,” and which without, we witness once again the vacillation between the Weak and Strong PCE.

In the end, I submit, Kim's argument moves in a circle.  Mental events, on a physicalist functionalist scheme, supervene on physical events, and so are dependent.  What is it about causality that prevents a mental event from causing a certain effect?  Kim answers: there is a physical (non-supervening, hence independent) event that causes that very effect.  But why can't an independent and a dependent event each cause the effect?  Kim answers: because the causal exclusion principle rules that out.  Well, the Strong PCE does, but not the Weak PCE.  And the Strong PCE just is the statement that a dependent event – such as a supervenient mental event – can't (also) cause the effect.  Attempts to break out of the circle, by providing independent justification of the Strong PCE over the Weak PCE, fail: so far as we can tell, all of the undisputed phenomena that the Strong PCE saves, the Weak PCE does as well.  So the Strong PCE – that a second (dependent) event cannot cause an effect – must simply be added (i.e., stipulated) as an additional premise in Kim’s argument for functionalist epiphenomenalism.  But then the argument is clearly uncompelling, since the added premise (the Strong PCE) effectively is the conclusion.

In addition, note that by categorizing causes in terms of  their completeness and (particularly) their independence, we can now understand why, in my earlier study (Beakley forthcoming), attempts to causally exclude the supervenient mental, in terms of various forms of causation and causal exclusion, ended in failure. We apply principles of causal screening off only to causal candidates that are prima facie independent  – the barometer needle versus atmospheric pressure with respect to rain, not to mention birth control pills versus John Jones’ maleness with repect to his failure to become pregnant, or a child’s utterance of “Abra Cadabra” versus his interaction with an electronic detection device with respect to the opening of the supermarket door.  And since the Strong PCE does not avail itself of the concept of “independence” (including supervenience as a form of dependence), we can rephrase the moral drawn in that earlier paper: the Strong PCE, but not the Weak PCE, is stymied in the face of those failures of the physical to screen off the supervenient mental.  So perhaps we do have a case that can decide between the Weak and Strong PCE after all.

Finally: my conclusions rest on a substantive assumption, viz., that I have indeed found the most reasonable interpretation of Kim’s statements, and particularly of his core terms – the “complete” and “independent” of his “complete, independent explanation,” and the causal exclusion principle built out of this.  I admit, I do not know how to prove conclusively that there is no other possible interpretation Kim could offer, which likewise fits his declarations on causal exclusion, but avoids the problems I have posed.  But, on the other hand, there is only one way for Kim to prove that such an alternative does exist, and that is by providing it.  Certainly his arguments for functionalist epiphenomenalism will not persuade unless the premises invoked are deemed uncontroversially true; and we cannot judge the truth of these premises, or the validity of his inferences, until we have fixed the meanings of the terms involved.  So my own reading of these terms will, at a minimum, stand as a challenge: a challenge to provide a more sympathetic articulation of these terms consistent with Kim’s numerous pronouncements on mental causation, an articulation which renders the premises of his argument both generally acceptable and sufficient to entail his conclusion.  In the face of that challenge, an implicit and unarticulated understanding of the core terms cannot be adequate to secure Kim’s conclusions.

VII. Conclusion.  In my earlier study (Beakley forthcoming) I argued that there appears to be nothing in going accounts of causation – “real causation,” mind you, not just “explanatoriness” – that rules out supervening (dependent) causes.  Here I have sought to demonstrate instead that Kim’s attempts to motivate the needed principle of causal exclusion, independently of any particular account of causation, ultimately proceed in a circle.  

Still, my ‘compatibilist’ conclusions about functionalist mental causation might seem particularly unsurprising.  Specifically, they might seem just the point Jackson and Pettit, and Block – and, at one point, even Kim – had already made about the different ‘types’ of  causation involved in mental causes, vis a vis “basic” physical causes.  In Jackson and Pettit’s parlance, functional properties have only “causal relevance,” whereas basic physical properties alone have “causal efficacy” (Jackson and Pettit 1990: 209).  The weaker relation of causal relevance hinges on Jackson and Pettit’s “invariance of effect under variation of realization, discussed earlier” (ibid.: 204).  In light of multiple-instantiability, one and the same sort of functional state can appear through a number of different ‘instantiation bases’; and if in each case the same sort of behavior ensues (as would hold, given their “causal-role” account of functional states), the invariant functional description picks out a common feature unrecognized in any of the different physiological accounts.  (Elsewhere they call this weaker sort of explanation “programming explanation” — where P is the relevant-but-inefficacious functional state and Q the explanandum event, “P causally explains Q by programming it, even though it may be that P does not produce Q” (Jackson and Pettit 1988: 392-4).)  Here, they explain, the functional account’s “explanatory interest will lie in the fact that it tells us about what would happen [under different situations] in addition to what did happen.  This is how the content properties may be causally relevant” (Jackson and Pettit 1990: 206).  So the principle of causal-explanatory exclusion is mitigated: while only one set of properties can be causally efficacious, more than one set of properties can be causally relevant.  And by broadening causal relevance in this manner, Jackson and Pettit feel they are making a case against epiphenomenalism.  For, as they recognize, functional properties of cognition will have every bit as much causal relevance as properties of any other science—except physics: “the functionalist account of content does not downgrade its causal role, rather it leaves it in the excellent company of everything except for certain members of that most exclusive of clubs, the properties of fundamental physics” (ibid.: 209-10).

But for terminological differences, this point is preserved entirely in Block’s work.  For Block, functional properties – and ‘second-order’ properties generally – can have “causal-explanatory relevance,” in place of the “causal relevance” (or “causal efficacity”) reserved for the lower (or lowest) level (Block 1990: 162).  But these two terms line nicely with Jackson and Petitt’s counterparts, since here again the quasi-causality of the higher-level property hinges on systematically following a genuinely causally relevant lower-level property:  
A causally inefficacious property can nonetheless be causal-explanatory if it “brings in” a causally efficacious property.  But the causally relevant property is the first-order one, not the second-order one (assuming that my argument for the standard causal inertness of the second-order was right).  (ibid: 163)

In fact, even Kim himself, while crediting only the physical with true causation, did at one time allow that supervenient properties are at least capable of “supervenient causation,” (Kim 1984b: 95ff.).  But of course, while he took his proposal of this relation to reserve an element of causation for functionalist mental states, in his later arguments for functionalist epiphenomenalism he downgrades his estimates on this prospect quite severely.

In each case, the ersatz relation rides systematically on the coattails of real causation, and hence reliably ‘tracks’ true causation, allowing a certain sense to be made of everyday attributions of causal powers to mental states.  But one point about this sort of  ‘solution’ must be kept well in mind: it concedes that mental states, on the functionalist outlook, do not have the same type of causation as do physical events – that is, we use the word “causation” only equivocally when speaking here of both mental and physical causation.  Hence, as concerns the sort of ‘genuine’ causation allegedly found among physical events or properties, all of the authors here concede that functionalist mental states do not have it; and the question then becomes only whether we can live with this consequence and remain functionalists, or must instead scrap functionalism and move to another view.

So if I have conceded that physical causes are “independent” causes of an event, while mental causes are ‘merely’ “dependent causes,” am I simply coining different phrases for the same distinction?  

That ecumenical reading overlooks a fundamental difference.  My claim is that the physical and mental events and properties are fundamentally different in one way – the physical are (in respect of supervenience) independent, whereas the mental are dependent.  But nowhere in my account do I suggest that there are two different “types,” or “strengths,” of causation, as all of these authors do.  On the contrary, my claim is precisely that the term “causation,” however it is interpreted, applies univocally to mental and physical causes, and that a proper understanding of mental causation has no need of any second-best, ersatz causality for mental events.  So my point – that the Weak PCE is all that’s needed for a physicalist functionalism – comes down just as strongly against both Block, and Jackson and Pettit, as it does against Kim: for we will feel the need to construct some second, surrogate “causation” for mental states only if it is taken for granted that they cannot bear the generic sort of causal relations to effects that physical events do. What is being taken for granted in that case is that, since the physical base event bears the causation relation to the effect, no other event can.  But that is just an uncritical embrace of the Strong PCE.  My conclusion, by contrast, is not that there is some palatable second kind of connection, as these apparently sympathetic authors suggest; it is rather that there is a (perfectly ordinary) second kind of event or property – what I am calling “dependent” events and properties.  But that in no way implies that such a dependent event cannot be a cause – in the very same sense of cause that applies to independent events.