Analysis 3 February 2013 (1)

7/28/2019 Analysis 3 February 2013 (1)

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Is Multipe Realization Coherent with Supervenience?

Meir Hemmo and Orly Shenker

According to a prevalent approach in the philosophy of mind the conjunction of token

physicalism, multiple realization and supervenience gives the best account of themental. There are various ways in which each of the above ideas is understood in the

literature. We shall construct a model that satisfies all of them thus showing that the

three ideas are mutually consistent. However, we will argue that the motivation foraccepting supervenience strongly undermines the motivation for accepting multiple

realization. In this sense, the two ideas are notquite coherent with each other.

Token physicalism as we understand it says that every particular mental state M1, for

example, a particular sensation of pain occurring at a certain time for a certain person,

is identical to a particular physical state (according to some physical theory, however

Hemples dilemma is solved), say X1 (see Figure). We are seeking an account of

token physicalism that can accommodate multiple realization, and therefore take it asa minimal requirement that there are in some sense mental kinds that can be multiply

realized. The question that immediately arises is then: what are the identity conditionsof mental tokens that belong to the same mental kind? A token state is characterized

(perhaps partially) by some indexicals such as the time and place of occurrence of thestate, and perhaps terms designating personal identity. Let us now consider the set of

all the tokens of a mental kind M, such that when stripped of their indexicals are

identical to each other. We shall take it that a minimal requirement for the idea that

there are mental kinds (that can be multiply realized) is that this set contains more

than one token. Consider now the set X of physical states which contains token

physical states Xi which are identical to each other when stripped of their indexical

terms. We take token physicalism to say that for every Mi there is a physical state

identical to Mi. For example, some of the Mi are pairwise identical to some physicalstate, for example Xi. In this example the intersection of X and M is not empty.

Whether or not one of them is a subset of the other is determined by whether multiplerealization or supervenience hold.

Consider now multiple realization. The idea is that a mental kind M is realized by

heterogeneous physical states, say states in X and in a set Y of different physical

states Yj: that is the physical state Yj, when stripped of its indexicals does not belong

to X but rather to some other set Y of physical states. Nevertheless, there is some

mental state Mj (j is not equal to i) such that when stripped of its indexicals Mjbelongs to M, and Yj is identical to Mj and thus realizes M. It follows that the setconsisting of the Xi and the set consisting of the Yj are each a proper subset of the set

M. Although our way of presenting multiple realization is not standard, it isessentially the same as the standard one.

Our formulation of multiple realization is deliberately meant to accommodate both

token physicalism and the idea that there are mental kinds that can be multiply

realized. But how different should the states in X and in Y be? If Xi and Yj share no

physical features at all, one may wonder which facts make it the case that Xi and Yj

both realize M-states (see also below): it cannot be physical facts (which we now

assume they dont share), and referring to mental facts as explanatory begs thequestion. Therefeore, accepting multiple realization amounts to accepting the fact that


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the Xi and the Yj are M-states as a primitive brute fact, which by definition cannot beexplained by reference to any other sort of fact. And so the fact that Xi and Yj are M-

states remains mysterious.

What about supervenience? Take a physical token state Yk (where k is not equal to j

or i), such that when stripped of its indexicals, Yk belongs to Y (see Figure). Take amental state N1 such that when stripped of its indexicals N1 is not identical to M, but

rather to some other mental kind N. We shall say that supervenience fails in this

picture, if Yk is identical to N1. Since the failure of supervenience is consistent withthe picture above in which both token physicalism and multiple realization hold,

supervenience is not logically entailed by them. If one wants to add supervenience asa further condition, the question arises what would be the motivation for it.

Supervenience is often motivated by the idea that that the mental should depend on

the physical in the sense that there could be no change in the mental (e.g. from M to

N) without a change in the physical (from Y to, say, Z). This is a necessary condition

for the idea that one can manipulate the mental by manipulating the physical (as isdone in the context of brain research or psychiatry, for example). However, if one

accepts supervenience it seems that there is no logical or set theoretic reason why oneshould accept multiple realization in the first place, namely that there could be a

change in the physical (say, from X to Y) without a change in the mental M. Ofcourse one may motivate the distinction between these two cases (and thus motivate

supervenience) by appealing to intuitions about the mind-body question and about the

nature of physics. But intuitions of this kind do not come from logic. We need

therefore to examine our motivation for supervenience, and once this motivation is

clear we need to go back to the picture above and examine whether our motivation for

supervenience can accommodate multiple realization as well.

Consider again the case in which supervenience fails. We have two physical tokensYj and Yk that differonly in their indexicals, that is, stripped of their indexicals they

are identical to each other and belong to Y (see Figure). Nevertheless if superveniencefails, Yj and Yk give rise to different mental states M and N (respectively). This

means that the only physical explanation of the difference in the mental is by

appealing to the difference in the indexicals of Yj and Yk. In other words, the mere

change of indexicals without any other change in the physical states brings about a

change of the mental. That is, the fact that Yj is identical to Mj at some time, place

and for some person is not invariant under temporal, spatial and personal translation.

Such failure of invariance is not a straightforward contradiction but it is highlyunreasonable given everything we know about the world in general and in currentphysics in particular (see more on this point below). And if a failure of supervenience

leads to the result that our mental states are determined by the indexicals only, thenwe believe that supervenience of the mental on the physical is highly motivated. We

take this argument to be the theoretically fundamental motivation for supervenience in

this context.

We now turn to examine whether the above motivation for supervenience can

accommodate the idea of multiple realization. And here we have a problem.

According to multiple realization as we described it above the set M consists of two

subsets Mi and Mj, such that every Mi is identical to the corresponding Xi but someMj are identical to Yj (see Figure). However, by construction, Mi and Mj, the


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(members of the) two subsets of M, differ only in their indexicals. And so the physicalstates Xi and Yj pick out the indexicals of Mi and Mj, rather than anything else. That

is, Xi is not identical to Mj just because of its indexicals. In other words, we have two

sets of physical states Xi and Yj that are different (not only in indexicals).

Nonetheless the Xi are identical to the Mi while the Mi are identical to Mj (except in

their indexicals), and the Mj are identical to the Yj which are different from the Xi(not only in indexicals). If this is true it means that the difference in the physical

content (everything except the indexicals) of the Xi and the Yj is not responsible for

the mental content of the M-states, but only to the difference in their indexicals. Onemay argue in response that the mental content of the M-states is fixed by what is

common to the Xi and Yj. But this cannot be true since by construction Yk shares thesame common features (it is the same as Yj in everything except indexicals) but is

nonetheless identical to N1.

Although the dependence of mental states on indexicals is not a straightforward

contradiction, it goes against everything we know in current physics. In some physical

theories indexicals are important, for example Aristotles theory concerning naturalpositions of the elements, Newtons conception of absolute space (to which Leibniz

objected), and in a different sense the Big Bang theory in cosmology. However, theroles of indexicals in these theories do not seem to us relevant to the issue, since they

are of a rather specific nature. We do not see any other reason for introducingindexicals in this role except the ad hoc need to accommodate multiple realization

with supervenience in philosophy of mind.

Our conclusion is that multiple realization and supervenience are not coherent with

each other in the sense that justifying either of them undermines the justification of

the other.

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Analysis 3 February 2013 (1)