The only serious argument
for incompatibilism that I know is the Consequence argument due, most famously,
to Peter van Inwagen. (An Essay on Free Will, OUP, 1983.)
The
version I will discuss is due to David Lewis. ("Are We Free to Break the Laws?”, Theoria 47 (1981), 113-121)
He
tells us to think of the argument as a reductio. A compatibilist
is someone who claims that the truth of determinism is compatible with the
existence of the kinds of abilities that we assume we have in typical situations
in which we deliberate and make a choice.
Let’s call these ‘ordinary abilities’. The Consequence argument claims
that if we suppose that a deterministic agent has ordinary abilities, we are forced to credit her with incredible abilities as well.
Here
is Lewis's argument.
Pretend
that determinism is true, and that I did
not raise my hand (at that department meeting, to vote on a proposal) but had
the ordinary ability to do so. If I had
exercised my ordinary ability – if I had raised my hand -- then either the
remote past or the laws of physics would have been different (would have to
have been different). But if that’s so, then I have at least one of two
incredible abilities – the ability to change the remote past or the ability to
change the laws. But to suppose that I have either of these incredible
abilities is absurd. So we must reject
the claim that I had the ordinary ability to raise my hand.
Van
Inwagen doesn't object to Lewis's way of stating his argument. On the contrary, he has said that Lewis's paper is “the finest essay that has ever been
written in defense of compatibilism – possibly the finest essay that has ever
been written about any aspect of the free will problem”. ("How to Think about the Problem of Free
Will”, Journal of Ethics (2008) 12,
337-341).
Van
Inwagen now agrees that the Consequence argument fails as a reductio.
However,
he claims that it has nevertheless succeeded in
"raising the price" of compatibilism. (Freedom to Break the Laws",
I
disagree. I say that the argument neither succeeds as a reductio nor succeeds
in "raising the price" of
compatibilism - that is, the price of commonsense at a deterministic world. What the argument does achieve -- at least on
Lewis's articulation of it -- is a clear statement of the counterfactuals to
which the compatibilist is committed. The argument is valuable for this
reason. It makes it clear that we need
to understand counterfactuals in order to understand what's at stake in the
free will/determinism debate. But as an argument for incompatibilism, it fails.
Lewis's
criticism of the Consequence Argument was published in 1981. His criticism was impeccable but his timing
was bad. Lewis had published Counterfactuals (his possible worlds
semantics and logic for counterfactuals) only 8 years earlier, in 1973, and
counterfactuals were still poorly understood, and apparently not understood at
all by some of the critics of Lewis's reply who seemed to think that Lewis had
invented "local miracles counterfactuals" for the express purpose of
defending a new and bizarre kind of compatibilism - "Local Miracles
Compatibilism". There was further
confusion due to the fact that Lewis developed his theory of counterfactuals in
two stages: the formal logic came first (in 1973); and it was not until 1979
("Counterfactual Dependence and Time's Arrrow") that Lewis proposed a
detailed similarity ranking for
possible worlds, and showed how to apply this similarity ranking in a way that
gets the right truth-conditions for counterfactuals. It was also well-known, by then, that Lewis
hoped to use counterfactuals to provide a counterfactual analysis of causation. All this was wildly ambitious, and many
people were skeptical that Lewis could pull it all off. But -- and this is my main point -- it was
natural, and understandable, back then,
to think that Lewis's theory of counterfactuals is a "package deal"
which you can accept only if you accept other parts of Lewisian metaphysics. So
many people hesitated, and this may explain why Lewis's critique did not have
the effect it should have had.
But
now that time has passed and the dust has settled, it's clear that this is not
the case. Lewis's theory of
counterfactuals is independent of most of his other views. You can accept his
theory of counterfactuals (including everything he says in response to the
Consequence Argument) without accepting any of the following: Lewis's
controversial brand of realism about
possible worlds, his counterfactual analysis of causation, his "Best
System" version of a Humean account of laws, his thesis of Humean
supervenience.
Furthermore,
Lewis's criticism of the Consequence Argument doesn't depend on the truth of his theory of counterfactuals. (His
theory is, I believe, correct, but even if it weren't his criticism would still
stand.)
Lewis's
formulation of the Consequence argument nicely highlights a point that the
better known modal version of the argument glosses over. The argument relies on a claim about counterfactuals. The argument says that
if determinism is true, then at least one of these counterfactuals is true:
Different
Past: If I had raised my hand, the remote past
would have been different (would have to have been different).
Different
Laws: If
I had raised my hand, the laws would have been different (would have to have
been different).
Now
I agree that both these counterfactuals strike many people as incredible. But there is a reason for that -- we are not used to thinking in terms of
determinism and we are not accustomed to counterfactual speculation about what would have to have been the case if
anything at a deterministic world had happened in any way other than the way it
actually happened.
On
the other hand, we are good at evaluating counterfactuals, or at least some
counterfactuals, and we are especially good at evaluating those counterfactuals
that we entertain in contexts of choice, when we ask questions about the causal
upshots of our contemplated actions.
(What would happen if... I struck this match, put my finger in the fire,
threw this rock at that window, raised my hand?) And when we contemplate our options, we take for granted the existence of many
facts - including facts about the laws and the past.
In
other words, when we evaluate
counterfactuals in real life, we do so by considering imaginary
situations which are very like the situation we are actually in, and we do not
suppose that there are any gratuitous departures from actuality. And to suppose a difference in the past or
the laws is a gratuitous difference -- if determinism is false.
So
it is no surprise that when our attention is directed to Different
Past and Different Laws, these
counterfactuals strike us as incredible, or at least odd. But that doesn’t mean that they are
false, and if determinism is true, then
either Different Past or Different Laws is true.
So
the first point is that we all need a theory of counterfactuals, and if
determinism is true, the true counterfactuals will include either Different Past or Different Laws.
The
second point is that the details of
the correct compatibilist solution to the free will/determinism problem will
turn on the details of the correct theory of counterfactuals.
If
David Lewis's theory of counterfactuals is correct, or even more or less correct, then the
relevant counterfactuals about the past and laws, at a deterministic world,
are:
1. Same
Past: If I had raised my hand, the past would still
have been exactly the same until shortly before the time of my decision.
2.
Slightly Different Laws: If I had raised
my hand, the laws would have been ever so slightly different in a way that
permitted the occurrence of a lawful divergence from actual history shortly
before the time of my decision.
On
the other hand, if Lewis's theory is wrong, and counterfactuals are always
evaluated by holding the laws constant, then the relevant counterfactuals, at a
deterministic world, are:
1. Same
Laws: If I had raised my hand, the laws would still
have been exactly the same.
2. Completely Different Past: If I had raised my hand, the past would have
been different all the way back to the Big Bang.
We've
got to choose. We need a theory of counterfactuals that applies at deterministic
worlds, and our choice is limited to a theory that accepts Slightly Different Laws or Completely
Different Past. Which theory we
choose has nothing to do with the free will/determinism problem and everything
with how we evaluate counterfactuals (in standard contexts).
Having
sorted this out, I will now explain Lewis's critique of the Consequence
Argument in a way that doesn't require you to accept the truth of Lewis's
theory of counterfactuals:
Lewis's
response to the Consequence Argument goes as follows: The argument trades on an
equivocation between two counterfactuals.
(C1) If I had raised my hand, the laws (or the
past) would have been different.
(C2) If I had raised my hand, my decision or
action would have caused the laws (or
the past) to be different
There
is a corresponding equivocation between two ability claims:
(A1) I have the ability to do something such that
if I did it, the laws (or the past) would have been different.
(A2) I have the
ability to do something such that if I did it, my decision or action would have
caused the laws (or the past) to be
different.
The
problem with the argument, says Lewis, is that it equivocates between these two
ability claims. To count as a reductio against the compatibilist, the argument
must establish that the compatibilist is committed to A2.
But the compatibilist is committed only to C1
and thus only to A1. The
compatibilist is committed only to saying that if determinism is true, we have
abilities which we would exercise only if
the past (and/or the laws) had been different in the appropriate ways. And while this may sound odd, it is no more
incredible than the claim that the successful exercise of our abilities
depends, not only on us, but also on the co-operation of factors outside our
control. Since we are neither
superheroes nor gods, we are always in this position, regardless of the truth
or falsity of determinism.
To
sum up: The Consequence Argument was
supposed to show that if we attribute ordinary abilities to deterministic
agents, we are forced to credit them with incredible past or law-changing
abilities as well. But no such
incredible conclusion follows. All that
follows is something that we must accept anyway, as the price of our non-godlike
nature: that the exercise of our abilities depends partly on circumstances
outside our control.