NATURALISM, EVOLUTION, AND TRUE BELIEF
Stephen Law
Plantinga’s
evolutionary argument against naturalism (EAAN) is currently one of the most
widely discussed arguments targeting philosophical naturalism (see, for
example, Beilby 2002). Plantinga aims to show that naturalism, in
combination with evolutionary theory, is, as he puts it, ‘incoherent or
self-defeating’. His argument turns crucially on the claim that, in the absence
of any God-like being to guide the process, natural selection is unlikely to favour
true belief. This, Plantinga supposes, is because natural selection selects
only for adaptive behaviour. It is irrelevant, from the point of view of
unguided evolution, whether the beliefs that happen to cause that adaptive
behaviour are true.
I argue
that, even in its most recent incarnation, the EAAN fails. In particular,
Plantinga overlooks the fact that adherents of naturalism may hold, seemingly
quite plausibly, that there exist certain conceptual links between belief
content and behaviour. Given conceptual links of the sort I envisage, natural
selection will indeed favour true belief.
I then
point out a further interesting, and perhaps somewhat surprising, consequence
of the existence of such conceptual links: that even if semantic properties such
as being a true belief are epiphenomenal – even if such properties have no
causal impact on behaviour – unguided evolution will still favour true belief.
The EAAN
For those
unfamiliar with the EAAN, here is a brief outline.[1] Let Naturalism (N) be the
view that there’s no such person as God or anything at all like God, and
Evolution (E) be the view that our cognitive faculties have come to be by way
of the processes postulated by contemporary evolutionary theory. Then, argues
Plantinga, the combination N&E is incoherent or self-defeating. This, he
maintains, is because if N&E is true, then the probability that R – that we
have reliable cognitive faculties (that is to say, faculties that produce a
preponderance of true over false beliefs in nearby possible worlds) – is low.
But, concludes Plantinga, anyone who sees that P(R/N&E) is low then has an
undefeatable defeater both for R and for any belief produced by their cognitive
faculties, including their belief that
N&E.
But why
suppose P(R/N&E) is low? Plantinga supports this premise by means of a
further argument. He begins by asserting that
materialism or physicalism is de rigeur for naturalism… A belief, presuming there are such
things, will be a physical structure of some sort, presumably a neurological
structure. (Forthcoming: 2)
According to a proponent of
naturalism, then, this structure will have both neurophysiological (NP)
properties and semantic properties. However, it is, claims Plantinga, unlikely
that the semantic properties of the neurological structure will have any causal
effect on behaviour:
It
is easy to see how beliefs thus considered can enter the causal chain
leading to behavior; current science gives us a reasonably plausible account of
the process whereby volleys of impulses propagated along the efferent nerves
cause muscle contraction, motor output, and thus behavior. It is exceedingly
difficult to see, however, how they can enter that chain by virtue of their content. A given belief, it seems, would have
had the same causal impact on behavior if it had had the same NP properties,
but different content. (Forthcoming: 2-3)
Plantinga
concludes that N&E makes semantic epiphenomenalism (SE) likely. But, says
Plantinga, if semantic properties such as having such-and-such content or being
true cannot causally impinge on behaviour,
then they cannot be selected for by unguided evolution. Given SE, truth and
falsehood will be, as Plantinga puts it, invisible
to natural selection. In which case, (on the modest assumptions that (i) 75% of
beliefs produced must be true in order for a cognitive mechanism to be reliable
and, (ii) that we have at least 100 such beliefs) P(R/N&E&SE) will be
low.
So runs
the EAAN. Recently, Plantinga has refined the argument by trying to tackle a
certain sort of objection. The objection is that by also embracing, for
example, reductive materialism (RM), adherents of naturalism may, after all,
quite reasonably suppose that they have evolved reliable cognitive faculties.
Why so? Well, on Plantinga’s understanding of RM, content properties just are NP properties. But then,
because NP properties cause behaviour, and semantic properties just are NP
properties, so semantic properties can cause behaviour. And if semantic
properties can cause behaviour, then they can, after all, be selected for by
unguided evolution.
Plantinga’s argument that P(R/N&E&RM)
is low
In his
most recent presentation of the EAAN, Plantinga attempts to deal with the above
objection. He focuses his attention on one semantic property in particular – truth.
Even supposing that semantic properties such as being true can causally affect
behaviour, why, he asks, should we suppose, that unguided evolution favour
beliefs that are true?
According
to Plantinga, the combination N&E&RM gives us no reason to suppose that
the content of belief/neural structures resulting in adaptive behaviour is
likely to be true. Suppose the belief/neural structure resulting in a piece of
adaptive behaviour has the content q.
While the property of having q as content does now enter into the causal chain
leading to that behaviour, it doesn’t matter whether q is true:
What matters is only that the NP property in question cause
adaptive behaviour; whether the content it constitutes is also true is simply
irrelevant. It can do its job of causing adaptive behaviour just as well if it
is false as if it is true. It might be true, and it might be false; it doesn’t
matter. (Forthcoming:10).
But if
the NP property can do its job of causing adaptive behaviour just as well
whether the content is true or false, true belief cannot be favoured by natural
selection. In which case, concludes Plantinga, (PR/N&E&RM) remains low.
Conceptual constraints on likely semantic
content
There is,
it seems to me, a fatal flaw in even this latest incarnation of the EAAN.
Plantinga
supposes that what unguided evolution favours, in the first instance, is
adaptive behaviour. As to what causes that behaviour, evolution doesn’t care.
True beliefs, false beliefs, something else - it’s all the same to evolution.
It is only the result – adaptive behaviour – that is preferred.
But even
if unguided evolution doesn’t care what causes adaptive behaviour, just so long
as it is caused, it may not follow, given certain further facts about belief that natural selection won’t also favour
true belief.
Consider
the suggestion that there exist certain conceptual
constraints on what content a given belief can, or is likely to, have given
its causal relationships to, among other things, behaviour. My claim is that,
given the existence of certain conceptual constraints, unguided evolution will
then tend to favour true belief.
To begin,
let me sketch out a simple illustration of how such constraints might
operate. Suppose we just stipulatively
introduce certain terms/concepts. Let’s say that a subject’s belief state has
content MC1 iff that state has
properties achieving a threshold of at least 30 points, with points allocated
thus:
Property
A +20 points
Property
B +15 points.
Property
C +20 points
Property
D -12 points
Notice
there’s no one property possession of which is essential if a state is to
qualify as having the content MC1. Suppose we similarly stipulate that a
subject’s belief state has content MC2
iff that state possesses properties achieving a threshold of at least 30 points,
with points allocated thus:
Property
D +20 points
Property
E +15 points
Property
F +20 points
Property
A -12 points
Note that
if a subject has a belief state with properties A and B, then, ceteris paribus, that state is rather
more likely to have the content MC1 than it is the content MC2 (though it might
yet turn out to lack content MC1 and possess content MC2 instead if it also
possesses properties D, E and F while lacking C). Now suppose that while not
all these properties involve causal links to behaviour, some do, namely A, C, D
and F. Property A is that of causing behaviour B1 in situation S1, C that of
causing behaviour B2 in situation S2, D that of causing behaviour B3 in
situation S3, and F that of causing behaviour B4 in situation S4.
Having
introduced these conceptual constraints on what it is to have beliefs with the
contents MC1 and MC2, we can now see how natural selection might select not
only for or against certain behaviours in certain situations, but also for or against these two belief contents.
Suppose that exhibiting B1 in S1 and B2 in S2 is in each case adaptive, while
exhibiting B3 in S3 or B4 in S4 is maladaptive. Then, other things being equal,
natural selection will tend to favour subjects holding beliefs with content BC1
over those holding beliefs with content BC2. So, given conceptual constraints
on belief content of the sort outlined above, natural selection need not be blind to belief content. It will
select for some contents over others, depending on the kinds of behavioural
output with which they are conceptually associated.
So now
suppose that constraints of this sort exist on the content of beliefs of the
sort with which we are already familiar – contents such as that there is water
five miles south, that Paris is the capital of France, and so on. Suppose these
constraints conceptually link content with behavioural output. No doubt these
constraints will be more complex than in my illustration. But, supposing they
exist, with what sort of behaviour is a given content likely to be conceptually
linked?
Suppose
that, solely in combination with a very strong desire for water, a certain
belief/neural structure typically results in a subject walking five miles to
the south. Surely, if there are such conceptual links between behaviour and
content, then the property of causing that behaviour in that situation will be
among those properties lending, as it were, a considerable number of points
towards that belief/neural structure achieving the threshold for having the
content that there’s water five miles south. Other things being equal, that
belief/neural structure is much more likely to have the content that there’s
water five miles south than it is, say, the content that there’s isn’t water five miles south, or that
there’s water five miles north, or that there’s a mountain of dung five miles
south, or that Paris is the capital of Bolivia. Perhaps the belief/neural
structure in question might yet turn out to have one of these other contents.
We can know a priori, solely on the basis of conceptual reflection, that, ceteris paribus, the fact that a
belief/neural structure causes that behaviour in that situation significantly
raises the probability that it has the content there’s water five miles south.
Among the various candidates for being the semantic content of the
belief/neural structure in question, the content that there’s water five miles
south will rank fairly high on the list.
But now
notice that, given such conceptual constraints exist, unguided evolution will
indeed favour true belief. Consider our thirsty human. He has a strong desire
for water. He’ll survive only if he walks five miles south to where the only
reachable water is located. He does so and survives. Suppose this adaptive
behaviour is caused by a certain belief/neural structure. If there are
conceptual constraints on belief content of the sort I envisage, and if a
belief/neural structure in that situation typically causes subjects to walk
five miles south, then it is quite likely to have the content that there’s
water five miles south – a true belief. Were our thirsty human to head off
north, on the other hand, as a result of his having a belief/neural structure
that, in that situation, typically causes subjects to walk five miles north,
then it’s rather more likely that the belief in question is that there’s water
five miles north. That’s a false belief. Because it is false, our human will
die.
So if
beliefs/neural structures cause behaviour, and if there are conceptual
constraints linking content with behavioural output of the sort I am
suggesting, then natural selection won’t just favour adaptive behaviour. It
will also favour true belief.
True,
there are other candidates for being the content of the belief that causes our
human to head off in the right direction. Perhaps some are more likely
candidates. Suppose our human has no conception of miles or south. Then,
instead of the belief that causes his behaviour having the content that there’s
water five miles south being, perhaps it has instead the content that there’s
reachable water thataway. However,
notice that, either way, the content of the belief in question is still true.
To sum
up: what Plantinga overlooks, it seems to me, is the possibility that there
exist conceptual constraints on content of the sort outlined here. The
suggestion is that if beliefs are neural structures, then it is at least partly by virtue of its having certain sorts of behavioural consequence
that a given neural structure will have the content it does. If such
constraints exist, then one cannot, as it were, plug any old belief content
into any old neural structure, irrespective of that structure’s behavioural
output. We run up against certain conceptual obstacles. If such conceptual
constraints exist, it appears natural selection will favour not only adaptive
behaviour, but also true belief.
Neither materialism nor functionalism not
presupposed
Note that
to suggest that such conceptual constraints on belief content exist is not, of
course, to presuppose that beliefs are neural structures or that materialism is
true. Let’s suppose, for the sake of argument, that substance dualism is true
and that beliefs are not neural structures, but soul-stuff structures. Then my
suggestion is that we may be able to know on the basis of a little conceptual
reflection that if beliefs are soul-stuff structures, and if a given soul-stuff
structure in combination with a strong desire for water typically results in
subjects walking five miles south, then ceteris
paribus that soul-stuff structure is quite likely to have the content that
there’s water five miles south, and is rather unlikely to have the content that
there’s water five miles north.
Also note
that to suggest that there exist conceptual constraints on content given
behavioural output is not to presuppose the truth of some reductionist,
materialist-friendly theory of content of the sort that Plantinga has gone on
to attack[2], such as Dretskian
indicator semantics or functionalism. Perhaps belief contents cannot be exhaustively characterized in terms of
their causal connections to input and output, as some functionalists claim.
That’s not to say that there are no conceptual constraints at all on what the content of a given belief is likely to be, given
the causal links that belief has to behaviour. Perhaps there are. Consider my
illustration involving contents MC1 and MC2. I stipulated that not all of the
weighted properties involved causal connections with behavioural output.
Properties B and E involved no such connections. Indeed, B and E might even be properties presenting an insurmountable obstacle
to any attempt to characterize the content of MC1 and MC2 in wholly
functionalist terms. It wouldn’t follow that there are no conceptual
constraints at all on beliefs having
content MC1 and MC2 given their behavioural output. Clearly there are.
So, while
the combination N&E&RM might be self-defeating, it seems that the
addition of CC – the thought that there are conceptual constraints on content
of the sort I envisage – produces a combination of beliefs that is not, after
all, self-defeating. It appears there are ways of embracing naturalism that
sidestep Plantinga’s charge of incoherence.
How natural selection can still favour true
belief even if SE is true
In fact,
it turns out that in order to sidestep Plantinga’s charge of incoherence our naturalist doesn’t even have to sign up
to RM. The addition of CC to R&E alone
is sufficient to rescue naturalism from self-defeat, as I’ll now explain.
As we saw
above, Plantinga’s initial worry about naturalism is that it makes semantic
epiphenomenalism (SE) likely. He supposes the naturalist will hold that beliefs
will be neural structures possessing both neurophysiological (NP) properties
and semantic properties. However, Plantinga thinks that only the NP properties
of those structures will then have any causal effect behaviour. A given belief
would have the same causal impact on behaviour if it had the same NP properties
but different semantic properties (or indeed no semantic properties at all).
So now
let’s suppose our naturalist actually bites the bullet and accepts SE – they
actually accept that the semantic properties of a given neurological structure
have no causal impact on behaviour. Plantinga supposes such a naturalist is
then compelled to accept that, because natural selection can only select for
adaptive behaviour and the properties that cause it, so natural selection
cannot select for the semantic property of being true. However, it turns out
that Plantinga’s assumption that natural selection favours only adaptive
behaviour and the properties that cause it is unwarranted. It turns out,
somewhat surprisingly, that, given CC, natural selection will still favour true
belief even if the property of being a true belief has no causal impact on behaviour.
To see
why, let’s return again to our thirsty human. He has a certain belief/neural
structure that, in conjunction his strong desire for water, causes him to walk
five miles south. Given the kind of conceptual constraints outlined above, a
belief/neural structure that causes a subject to walk five miles south given a
strong desire for water will quite probably have the content there’s water five miles south. Notice
it really doesn’t matter whether or not that belief/neural structure causes
that behaviour by virtue of its
having that semantic property. It remains the case that, if that sort of neural
structure for whatever reason has
that behavioural consequence, then, given CC, it quite probably has the content
there’s water five miles south and
probably doesn’t have the conceptual content there’s water five miles north. It matters not whether SE is true:
the behavioural output of a belief/neural structure still places constraints on
its likely content.
But then,
given such conceptual constraints, natural selection is likely to favour true
belief even if SE is true. Odd though
it might seem, given CC, natural
selection will favour true belief even if the property of being a true belief
has no causal impact on behaviour. This is a rather significant discovery,
even setting aside its relevance to Plantinga’s EAAN.
Conclusion
Of
course, I am merely making a suggestion. Perhaps there exist no such conceptual
constraints on belief content of the sort I envisage. Still, the view that
there are such constraints on content is widespread (it is by no means
restricted to those wedded to some form of logical behaviourism or
functionalism, for example). It seems intuitively obvious to many of us that
belief content is not entirely conceptually independent of behavioural output:
that one cannot plug any old belief content into any old neural structure (or
soul-stuff structure, or whatever) entirely independently of its behavioural
output. That intuition would appear to be, philosophically speaking, largely
pre-theoretical. It cannot easily be dismissed by Plantinga as a product of
some prior theoretical bias towards naturalism and/or materialism.
My
central conclusion, then, is this. Plantinga has not shown that naturalism in
combination with the theory of evolution is unavoidably self-defeating. It
appears that an adherent of N&E who also supposes CC is true can, after
all, quite reasonably suppose they have evolved reliable cognitive faculties.
In
response, Plantinga might now try to show that if naturalism is true, there are
unlikely to be conceptual constraints on semantic content of the sort I
describe. Perhaps he can do this. If so, then the EAAN might be resurrected.
But as things stand, it is not naturalism that is defeated, but the EAAN.[3]
Heythrop College,
University of London
London W8 5HN
References
Beilby, J. (ed) 2002. Naturalism
Defeated? Essays on Plantinga's Evolutionary Argument Against Naturalism.
Ithaca: Cornell University Press.
Plantinga, A. Forthcoming. Content and Natural Selection. Philosophy and Phenomenological Research. Currently
available on-line at Plantinga’s departmental webpage: http://philosophy.nd.edu/people/all/profiles/plantinga-alvin/documents/CONTENTANDNATURALSELECTION.pdf
Page numbers refer to the on-line
version.
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