Nonreductive Physicalism and Strict
Implication I. The Strict Implication thesis
Robert Kirk
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Department of Philosophy
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Robert.Kirk@nott.ac.uk
Nonreductive Physicalism and Strict
Implication
Robert Kirk
Do physicalists have to be reductionists? I have argued that a strong
kind of physicalism based on the ‘Strict Implication thesis’ can consistently
reject both eliminativism and reductionism (in any nontrivial sense: see II
below) [8, 9, 10]. Andrew Melnyk disagrees. He faces me with this dilemma:
either my formulation doesn’t actually entail physicalism, or it must be
interpreted in such a way that the mental is after all reducible to the physical
[12]. His alternatives depend on two interesting assumptions, each worth
discussing for its own sake. One is about the metaphysical implications of my
formulation of physicalism, the other concerns what it takes to establish the
strict implication thesis. Each assumption turns out to be mistaken. The
outcome, I think, is that this kind of nonreductive physicalism becomes both
clearer and more clearly defensible.
The Strict Implication thesis is stated on the assumption that we have an
idealized version of today’s physics, including a set of laws and a special
vocabulary. Let P be a conjunction of all truths statable in terms of
that theory, including statements of physical laws together with all truths
about the distributions and states of elementary particles, or whatever the
fundamental items may be, throughout space and time. So P describes the
entire physical universe (assuming our idealized physics is successful). By
definition the special physical vocabulary does not include psychological
expressions, so P does not include any statements of any psychophysical
laws or true generalizations. Now consider whatever truths in psychological
language there may be (scientific and everyday, particular and general), and let
Q be a conjunction of all such actual truths.
(1) P strictly implies Q. That is, it is impossible
that P should be true and Q false.
It is crucial that the
impossibility involved here is the strongest kind there is: absolute. If you
suspect there is something not quite as strong yet still not merely natural or
nomic – ‘metaphysical’ necessity, perhaps – it is not what is in question
here.2 Note also three
further points. First, the Strict Implication thesis alone is not supposed to be
sufficient for physicalism. It has to be supplemented by two other
theses:
(2) Q does not strictly imply P;
(3) Nothing
exists other than what is strictly implied by P.
(2) and (3)
combine with (1) to ensure that it is the physical realities which determine the
mental, not the other way round. (3) is needed to rule out any conceivable
nonphysical items – ghosts, thought-rays and so on, on top of the purely
physical world.3
(More on this later.)
Second, although theses
(1)-(3) are individually necessary and jointly sufficient for minimal
physicalism, they cannot be regarded as providing a solution to the mind-body
problem. They leave it unexplained how the Strict Implication thesis can be
true, assuming it is. I see that as a virtue of the formulation (1)-(3). It
helps to make clear what philosophical work still has to be done.
Third,
theses (1)-(3) are not intended to say all there is to be said by way of
explanation of minimal physicalism. They are not even intended to express
directly the basic idea of physicalism. The basic idea is probably best conveyed
by statements like ‘Nothing exists but the physical’, or ‘Mental language is
just a special way of talking about certain parts and features of the physical’.
Given that basic idea, (1)-(3) are intended to make reasonably clear what
minimal physicalism is committed to. I will argue that theses (1)-(3) entail the
basic idea because they entail that the contingent psychological descriptions in
Q are redescriptions of certain parts and features of what’s described by
P, and apply in virtue of the latter (section
III).
II. Reduction
Kim argues that ‘a physicalist has only two genuine options,
eliminativism and reductionism’ [6, p. 267]. But ‘reduction’ is a slippery word.
Some philosophers – David Chalmers for example – seem to use it in such a way
that any position that I would count as even minimally physicalist would be ipso
facto reductionist.4 That
would trivialize Kim’s claim and make my position untenable from the start.
Obviously I am not maintaining that physicalists can avoid being reductionists
in that minimal sense. Kim is trying to show that physicalists are committed to
a variety of reductionism that is problematic for them (indeed
embarrassing: something that warrants the characterization of the reductionist
as one over whom there hangs ‘an aura of doctrinaire naiveté’ [6, p. 266]).
Accordingly, in [10] I attempted to show how noneliminative physicalists could
avoid any kind of reductionism which required the terms or explanations of the
reduced science to be correlated with those of the reducing science. Both
Kim’s and Melnyk’s arguments seem to presuppose that what is in question is that
‘classical’ type of reductionism. Later I will try to make clear that there are
no reasons to suppose (1)-(3) commit one to other nontrivial kinds of
reductionism either (section IV).
But I am not here concerned to attack
reductionism. My line is that no good reasons have been offered why physicalists
should be forced to accept it in any nontrivial sense, while there are some good
reasons to reject it (see [10] and section IV below).
It may seem misleading
to describe the physicalism represented by theses (1)-(3) as ‘nonreductive’,
since the expression ‘nonreductive physicalism’ is sometimes regarded as
equivalent to ‘property dualism’.5 But I think theses (1)-(3) provide a neat way to distinguish between
genuine physicalism and property dualism. If the argument in the next section
succeeds, then if you accept those theses your position can properly be
described as full-bloodedly physicalist – even if you reject nontrivial kinds of
reduction. If on the other hand you reject (1)-(3) – in fact, if you reject only
the Strict Implication thesis – you are committed to dualism or some more
extreme anti-materialist position, since you imply that psychological properties
involve something over and above the physical – you imply that P ‘leaves
something out’.6 If I am
right, theses (1)-(3) offer a genuinely physicalist alternative to
property dualism.
III. Why theses (1)-(3) entail physicalism
Melnyk argues that if I reject reductionism I am forced to construe
(1)-(3) in such a way that they do not entail physicalism. That is the first
horn of his dilemma. In his attempt to impale me on it he points out that there
would be no problem with an absolutely necessary link from P to Q
in (1) if it depended on either syntactic entailment or bridge laws, but notes
that both such explanations would entail reduction, contrary to my claim. The
only other possibility he can think of is that the relevant notion of necessity
is ‘primitive and unanalysable’. His key thought is that we could construe the
strict implication relation as ‘the holding of some primitive and unanalysable
necessary connection between properties of two otherwise quite distinct kinds’
([12], p. 324). He thinks it follows that on that interpretation theses (1)-(3)
do not entail physicalism because he assumes that:
(M1) . . . what makes
ascriptions of mental properties true is simply the distribution of mental
properties and those alone; on the current interpretation, nothing in the
conjunction of (1) through (3) guarantees that the distribution of physical
properties is in any sense doing so ([12], p. 325).
That is the first of the
two assumptions I mentioned at the start. Melnyk attempts to reinforce the
appeal of this assumption (M1) by remarking that ‘There seems no logical
difficulty in the idea of the weirdest, most intuitively non-physical
spook-stuff you like still being absolutely necessitated, in the sense of the
current interpretation, by the physical’ (pp. 325f.). That remark might have
seemed justified if my position had been a variety of property dualism. However,
even in that case I don’t see how the necessity could have been absolute. Surely
the dualist cannot claim more than that mental properties are nomically
linked to physical ones. Since, in contrast, the necessity of the link from
P to Q is absolute, (1)-(3) entail that it is not even logically
possible for the properties specified by P to be present and those
specified by Q to be absent. It is not as if any arbitrary pair of
properties could consistently be claimed to be linked by absolute
necessity.
Against assumption (M1) I will argue that (1)-(3)
entail:
(K) The contingent truths in Q are redescriptions of
certain parts and features of what’s described by P, and hold in virtue
of the latter.
(K) means, I take it, that (1)-(3) entail that the
contingent psychological truths in Q (in contrast to any logical or
conceptual truths there may be) are not just different ways of talking about
some of the realities (things, events, properties, whatever) described by
P, but made true by the latter. If that is right, the necessity
involved is far from being blankly ‘mysterious and unanalysable’. It is just the
familiar sort involved in any case where what is described in one way also
qualifies – purely because it fits that first description – for being described
in another way. Consider for example a small piece of quartzite, worn smooth in
a river-bed. P includes its own descriptions of this piece of rock,
together with accounts of the processes that resulted in its coming into
existence and rolling around in the river; but P’s narrow vocabulary does
not include the word ‘pebble’ itself. Obviously, though, the facts that P
states in its own terms make the description ‘pebble’ true in this case. The
necessity by which that description applies, given P, is surely not
mysterious; and if what follows is right, theses (1)-(3) ensure it is by the
same necessity that Q holds, given P.
What Melnyk needs to
support his assumption (M1) is cases of mental property-instances that are
absolutely necessitated by physical ones (as follows from theses (1) and (2)),
while their descriptions in Q are not just redescriptions of some
of the physical realities described by P. That is, he needs the following
two conditions to be capable of being satisfied simultaneously:
(A) The
realities (things, properties, whatever) described by P – the
P-realities – absolutely necessitate those described by
Q;
(B) The P-realities do not make Q
true.
(Recall that the P-realities make up a purely physical
universe. If they alone sufficed to make Q true, then given (2) and (3),
physicalism would be true.)
But it is impossible for (A) and (B) to be
satisfied simultaneously. Since Q includes contingent statements about
creatures’ mental states, something must make it true – the
‘Q-realities’. But (B) entails that those Q-realities include
something additional to the purely physical world described by P. We
could put this by saying that (B) entails that at least some of what Q
describes is extra stuff – which will of course be nonphysical
(‘spook-stuff’). The crucial point now is this. If the Q-realities
include nonphysical extra stuff over and above the P-realities, then
P alone, which is in purely physical terms, cannot logically imply either
what that extra stuff is, or even that it exists at all. A purely physical
description of the world cannot by itself settle whether there is anything
nonphysical. Hence (B) entails that no contradiction would be implied by saying
that the world could, logically, have been such that P was true and
Q false: there would have been the same purely physical
P-realities, but some or all of the actual Q-realities would have
been absent. But that contradicts (A). If it is so much as logically possible
for P to be true when Q is false, then P doesn’t strictly
imply Q. So the situation envisaged is impossible.
If theses (1) and
(2) hold, (A) holds; but we have just seen that in that case (B) cannot also
hold. Since by thesis (3) there is no extra stuff, (K) follows: Q not
only redescribes the P-realities, but is made true by them. So theses
(1)-(3) do after all entail physicalism.7
IV. Strict implication without reduction
FOOTNOTES REFERENCES
As remarked earlier, I see (1)-(3) not as a solution to the mind-body
problem but as a useful way to make reasonably clear what philosophical work has
to be done on it. We still need explanations of just how and why the Strict
Implication thesis holds; and such explanations are not themselves part of
(1)-(3). Melnyk concedes that such explanations ‘promise to go a useful way
toward explaining (1) [the Strict Implication thesis]’ [12, p. 326]. He also
notes that I claim they would require no appeal to psychophysical laws and would
not entail reductionism, but argues that I am wrong. I think his reasoning fails
because it rests on a second undefended assumption, which I will try to show is
also mistaken. This is:
(M2) The relevant explanations of why P
strictly implies Q must be ‘valid derivations of what is to be explained’
[p. 327].
Specifically he assumes that if P strictly implies
Q, then an explanation of the kind I envisage must consist of a sequence
of conditionals of the form:
A[P —> S]
A[S —>
Q]
(where ‘A’ is to be read as ‘It is absolutely necessary that . .
.’), from which a conclusion of the form A[P —> Q] can be seen
to follow. He also assumes that the first premiss in such a sequence must be
‘expressed, consequent and all, exclusively in the proprietary vocabulary of
physics’ (p. 328). Given those assumptions, he argues that sooner or later a
psychophysical bridge law will appear in the sequence of conditionals, from
which he reasonably infers a commitment to a correlation-based
reductionism.
It may be true that such reductionism follows from my position
given his assumption (M2). But I see no reason to accept (M2) (see [8, 9, 10]).
To illustrate the main points, consider the case of shapes. Recall that P
is a conjunction of all the true statements that are expressible in the
narrow vocabulary of physics, including statements of physical laws, and let
S be a conjunction of all truths about the shapes of things, including
both particular and general statements. Does P strictly imply S?
Well, we have a rich vocabulary for describing shapes, much of it depending on
comparisons, but some of it exploiting geometrical concepts. ‘Spherical’ and
‘regular tetrahedron’, for example, belong to the narrow physical vocabulary, so
many truths about things’ shapes will already be included in P and thus
be trivially strictly implied by P. But vague shape-descriptions such as
‘jagged’, ‘gnarled’ and ‘spoon-shaped’ are common, and surely occur in true
statements. Such statements will belong to S but not to P. Yet
surely they too are strictly implied by P, since it is pretty obvious
that:
(C) A thing’s having a certain shape involves nothing over and
above what is strictly implied by P.
In contrast, it is far from
obvious that:
(D) All truths about the shapes of things, including those
using expressions like ‘jagged’, ‘gnarled’, and so on, are correlated
with truths expressible in narrowly physical terms.
Yet if Melnyk is
appealing to a general principle, as he seems to be, it would have to be one
according to which the only way to establish (C) would be via (D). In fact it is
easy to see that we can establish (C) without establishing (D) – indeed even if
(D) is false. If anyone were so confused as to doubt (C), we need only point out
that:
(E) The shapes of things are just the arrangements of their
surfaces, which depend (contingently) on the distributions and states of the
physical particles which compose them.
It is hard to see how anyone could seriously dispute (E) (waiving
general Quinean qualms about ‘contingently’). As to what makes (E) true,
No more need be said. (E) doesn’t require us to
formulate detailed particular statements of the form A[P —> S]. It
doesn’t even entail that some relevant statements of that kind are true, though
hard to discover. (E) is entirely sufficient to make clear that the strict
implication in question holds – and it does so without providing a
definition of ‘shape’ in physical terms, and without appealing to
correlations, derivations, or reductions of any (nontrivial)
kind.8
That should not be surprising. The vocabulary of shapes leaves open
how things come to have the shapes they do; so when we describe things as having
such and such shapes, we are not thereby committing ourselves to the underlying
physical details. Indeed, treating shapes as if they were essentially linked
with the physics of the actual world would overlook that things could perfectly
well have had shapes even if the laws of physics had been different.
Nevertheless, it is the actual physical facts which fix (nomically) the
arrangements of things’ surfaces, and the facts about their surfaces which fix
(logically or conceptually) the facts about their shapes. That example
illustrates the point that there is no universal principle requiring cases of
strict implication by P to involve derivations of the type Melnyk
assumes.
Nor does there seem to be any reason why the psychophysical case
should be an exception. Some versions of behaviourism and functionalism would,
if successful, explain how it is that P strictly implies Q –
without recourse to derivations. To achieve that result, of course, they must
avoid claiming that each individual mental statement has necessary and
sufficient conditions in terms of behaviour or its causes: that claim entails
reductionism. But it has long been recognized that there is no need to insist on
such conditions: nondefinitional versions are available (see for example
Armstrong [1], pp. 84f.). Such nondefinitional varieties of behaviourism or
functionalism are analogues in the psychophysical case to (E) in the case of
shapes. If successful, they would explain why the Strict Implication thesis
holds without dependence on correlations.
You might suggest that unless
explanations of the deductive form in question were at any rate available, we
could not be justified in claiming that the strict implication held. But why
must that be so? (E) fully explains innumerable particular instances of strict
implication, and can be seen to cover them regardless of the fine details of
different particular shapes. To see that P strictly implies all truths
about shapes we do not have to consider each particular ascription of a shape to
an individual thing.9 We can
know (E) even though we know neither all the relevant physical facts, nor all
the facts about shapes. It would be pointless to try to discover the detailed
physical facts which held in particular cases, and it would probably obscure
what mattered.10
Although
Melnyk seems to have been thinking of the ‘classical’, correlating type of
reductionism, the above considerations have wider implications. Instead of
looking for bridge-laws or sentence-correlations, some types of reductionism aim
to construct an image or model of the reduced theory in the reducing theory – a
model deducible from the reducing theory conjoined with certain special
conditions (see e.g. [2], [4]). The idea is that once a reduction of that kind
has been achieved, ontological questions can be decided on the basis of how
‘smooth’ or ‘bumpy’ it is: how closely the categories of the reduced theory are
modelled in the reducing theory. But there is no more reason to suppose that
(1)-(3) commit us to that type of reductionism than to the classical type.
Nondefinitional functionalism has no more need to engage in the project of
constructing a model of psychology in physics (or in neurobiology) than it has
to take on the unpromising task of specifying necessary and sufficient
conditions for each individual type of mental state.11
Certainly physicalists owe
explanations of why everyday and/or scientific psychology work as they do, and
also of why the Strict Implication thesis holds. And of course reductionism of
whatever kind – provided it does not just eliminate psychology – would be one
way of providing such explanations. But the examples of nondefinitional
behaviourism and functionalism show it is not the only way. Neither Kim nor
Melnyk, nor anyone else to my knowledge, seems to have provided sound reasons
for thinking that physicalists have to be reductionists (or else eliminativists)
in any nontrivial sense.
Melnyk ends by arguing that theses (1)-(3) do not
adequately formulate physicalism about nomic mental facts. He rightly
notes that I must hold that P strictly implies every mental law (p. 331),
but alleges that I give no indication ‘either how or even that these
explanations [viz. the philosophical explanations which perform the role taken
by (E) for the special case mentioned above] could be used to explain the strict
implication by P of mental laws’. He goes on to claim that I am again
‘forced back to the interpretation of the strict implication of mental laws by
P’ in terms of ‘some primitive and unanalysable relation of absolute
necessitation of mental laws by the physical facts’ (p. 331).
I can now
reply very briskly. In offering (1)-(3) as a way of helping to make the
commitments of minimal physicalism reasonably clear, I did not also set out to
provide explanations of the kinds needed to establish that the Strict
Implication thesis does in fact hold. That is a different project (although I
have offered a contribution towards it in [8]). Sure enough, Q includes
all true laws and lawlike statements of scientific and everyday psychology.
Explaining how the facts specified by P make those statements true is a
nontrivial project; but if I am right, there are no good reasons to suppose it
cannot be done without resorting to reduction. When Melnyk claims I am ‘again’
forced back to a blankly mysterious type of necessity, that is because he again
overlooks that we can be justified in accepting that a relation of strict
implication holds without having to provide explicit derivations.
I have
argued that theses (1)-(3) entail that the contingent truths in Q are
redescriptions of, and made true by, parts and features of the world described
by P, so that they entail full-blooded physicalism, not mere property
dualism. (1)-(3) offer a reasonably clear alternative to property
dualism. Melnyk’s contrary assumption (M1) turned out to have inconsistent
implications. I have also argued (in [10] supplemented by this) that (1)-(3) do
not commit us to reductionism in any nontrivial sense: Melnyk’s objection
depends on a second mistaken assumption (M2). I think those conclusions enhance
the usefulness of (1)-(3) for making the commitments of minimal physicalism
reasonably clear.12
1. In earlier formulations I confined Q to truths about the
organisms whose existence is provided for by P. In view of (3) below that
restriction seems unnecessary.
2. For more detailed exposition and
discussion see [8], pp. 71-86, and [9]. For a similar approach see Frank
Jackson’s [5].
3. You might think (3) alone would be enough for
physicalism, but there are two reasons for including (1) and (2). One is that
(3) by itself would not rule out, as (2) does, a variety of idealism according
to which all physical truths were strictly implied by psychological truths. The
other is that just asserting that nothing exists but what is strictly
implied to exist by P fails to make explicit, as (1) does, that all
psychological truths – including e.g. any laws there may be – are
strictly implied by P. You might now be tempted to suggest that (3) would
be redundant if (1) were revised by substituting something like ‘all truths’ for
‘Q’. The trouble is that P alone does not strictly imply that
there are no nonphysical items unless, like Hobbes, you think materialism is in
any case necessarily true. If, like me, you think physicalism/ materialism is
not absolutely necessary, the suggested revised version of (1) would be false.
There is a notable contrast with one type of supervenience (though certainly not
with other types): the one Chalmers calls ‘logical’ supervenience ([3], ch. 2).
If in his sense you say ‘All facts/ entities/ properties logically
supervene on physical ones’, you automatically rule out nonphysical facts etc.
(I have argued that essentially that type of supervenience is the only one
consistent with physicalism: see [9], especially pp. 250-2.)
4. Chalmers
says, ‘A phenomenon is reductively explainable ... if the property of
exemplifying that phenomenon is globally logically supervenient on physical
properties’ [3], p. 48. Since his notion of logical supervenience is to all
intents and purposes equivalent to theses (1)-(3), that seems to make exponents
of (1)-(3) reductionist by definition. However, he distinguishes between
reduction and reductive explanation, so perhaps my interpretation is open to
question. Others are more explicit: e.g. Robert McCauley states: ‘Physicalism in
the philosophy of mind anticipates a comprehensive reduction . . .’ [11],
p. 712. Note that in [8] and [9] I have argued that all physicalists are
committed to something close to theses (1)-(3).
5. ‘Opposed to reductive
physicalism is nonreductive physicalism, also called property
dualism’ – Kim [7], p. 645. Cf. also Bickle: ‘. . . contemporary
nonreductive physicalism is identical to property dualism, traditionally
conceived. Much current “nonreductive physicalism” is not physicalism at all’
[2], p. 8.
6. For reasons, see [8], pp. 71-80, [9], pp. 244-9.
7.
Here I must mention another objection Melnyk alleges against my version of
minimal physicalism. He says, ‘Kirk’s physicalism is ... consistent with the
possibility of the world’s being exactly as it is physically, save in some
utterly trifling respect ..., but entirely lacking in mentality’ [12], p. 325,
n. 3. That is the ‘rogue atom’ objection advanced by Kim e.g. [6], p. 277. In
[10] I explain why it fails. Briefly, it overlooks the likelihood that what goes
on in remote regions of space is mostly irrelevant to mentality. If that is
right, the possibility Melnyk alleges is not genuine: many different
conjunctions of statements other than P will also strictly imply
Q, including vast numbers which assign different positions to atoms in
deep space.
8. See also [8, 9, 10]. I need hardly add that there is no
guarantee that all explanations of strict implication which do not
include derivations of the sort Melnyk assumes will be as straightforward as
(E).
9. Melnyk bases his reasoning on the assumption that the ‘valid
derivations’ he envisages will be about such particular ascriptions.
10.
These points are closely related to those made by Putnam about causal
explanations in [13].
11. It is worth noting that Bickle, in his
exposition of ‘new-wave reductionism’, regards functionalism, though
unattractive, as a nonreductionist option for physicalists (see e.g. [2], p.
48), thereby committing himself to the view that physicalism does not actually
entail either reductionism or eliminativism.
12. Thanks to Robert Black,
Ed Lindon, and Andrew Melnyk for comments on earlier versions, and special
thanks to Bill Fish for extended discussions. Thanks too to two anonymous
assessors for the AJP.
1. Armstrong, D. M., A Materialist Theory of the Mind (London:
Routledge and Kegan Paul,1968).
2. Bickle, J., Psychoneural Reduction:
The New Wave (Cambridge, Mass., and London: MIT Press, 1998).
3.
Chalmers, D., The Conscious Mind: in search of a fundamental theory (New
York and Oxford: Oxford University Press, 1996).
4. Churchland, P. S.,
Neurophilosophy: Toward a Unified Science of the Mind-Brain (Cambridge,
Mass. and London: MIT Press, 1986).
5. Jackson, F., From Metaphysics
to Ethics: A Defence of Conceptual Analysis, (Oxford: Clarendon Press,
1998).
6. Kim, J., ‘The Myth of nonreductive Physicalism’, in his
Supervenience and Mind: selected philosophical essays (Cambridge:
Cambridge University Press, 1993) pp. 265-284.
7. Kim, J., ‘Physicalism’,
in [14], p. 645.
8. Kirk, R., Raw Feeling: a philosophical account of
consciousness (Oxford and New York: Clarendon Press, 1994).
9. Kirk,
R. ‘Strict Implication, Supervenience, and Physicalism’, Australasian Journal
of Philosophy 74 (1996) 244-56.
10. Kirk, R., ‘How Physicalists Can
Avoid Reductionism’, Synthese 108 (1996) 157-170.
11. McCauley,
R., ‘Reductionism’, in [14], 712-14].
12. Melnyk, A., ‘The Prospects for
Kirk’s nonreductive Physicalism’, Australasian Journal of Philosophy 76
(1998) 323-332.
13. Putnam, H., ‘Philosophy and Our Mental Life’, in his
Mind, Language and Reality, Philosophical Papers, ii (Cambridge:
Cambridge University Press,
1975) 291-303.
14. Wilson,
R. A. and F.C. Keil (eds.) The MIT Encyclopedia of the Cognitive Sciences
(Cambridge, Mass. and London: MIT Press, 1999).
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