Ted Warfield has shown that the famous Consequence argument of his colleague Peter van Inwagen contains a modal fallacy by appealing to premises that are merely contingent. Van Inwagen and Robert Kane therefore only establish what Warfield calls WEAK versions of the incompatibilist thesis.
Warfield corrects this problem with a new "formal" argument for incompatibilism. His argument has a "formalistic" or "proof-like" appearance (as had van Inwagen's). But Warfield cautions that this appearance "should not lead anyone to think that I am trying in any literal sense to provide a proof of incompatibilism (or anything else)." Rather, he says, he simply puts forward a strengthened Consequence argument that is a "valid" argument where existing arguments were invalid because of the modal fallacy. He says
So far as I aware, all versions of the Consequence argument employ a "conditional proof" strategy; many also employ the terminology. Indeed, in most incompatibilist arguments the overall form of the argument is that of conditional proof. The argument typically looks like this: assume determinism and show that, given the assumption, no one has freedom. Most incompatibilists, however, either do not adequately understand or simply fail to adhere to a restriction relevant premises in such an argument must meet if the incompatibilist conclusion is to follow from such an argument. Most incompatibilists, to be precise, seem unaware that in order to get the incompatibilist conclusion that determinism and freedom are strictly incompatible (that no deterministic world is a world with freedom), their conditional proofs must not introduce or in any way appeal to premises that are merely contingently true in between the assumption of determinism and the step at which the "no freedom" conclusion is reached.Warfield had argued earlier that van Inwagen's "Mind argument" (named after the journal that had published the argument), showed that free will and indeterminism were incompatible. In a 1998 Mind article he wrote,
Libertarians believe that free will exists and is incompatible with determinism. Among the many problems facing libertarians is the problem of the alleged incompatibility of free will and indeterminism. If free will is, as many have suggested, incompatible with indeterminism then libertarianism is false. Libertarians have not adequately addressed this issue to date. It is this gap in the libertarian program that we seek to fill. We will show that the strongest argument for the incompatibility of indeterminism and free will, the so called Mind argument,1 is a failure. We will also, in providing an improvement on Peter van Inwagen's well known Consequence argument, show that the failure of the Mind argument does not threaten the strongest libertarian argument for the incompatibility of free will and determinism. In his widely influential book An Essay on Free Will (1983), Peter van Inwagen presented, among other arguments, his Consequence argument. This argument is, we think, the strongest argument to date for incompatibilism, the thesis that free will and causal determinism are incompatible. Unfortunately, as van Inwagen saw and as we will discuss below, an argument quite similar to van Inwagen's Consequence argument, the Mind argument, seems to show that free will is also incompatible with causal indeterminism. If both van Inwagen's argument and the Mind argument are sound then there is no such thing as free will and libertarianism is false.Van Inwagen's Mind argument is clearly the Like van Inwagen we are libertarians and wish to avoid this conclusion. Unlike van Inwagen, however, we have a satisfactory response to the challenge of the Mind argument. Our response, in brief, is to deny the soundness of both van Inwagen's Consequence argument and the Mind argument. After showing why both arguments are unsound, we will offer an improved version of the Consequence argument and show that the Mind argument cannot be similarly improved. We conclude that the Mind argument (and with it general worries about the incompatibility of indeterminism and free will) is no threat to the libertarian. Consider an indeterministic world in which, most importantly, the actions of agents are indeterministic consequences of agents' particular sets of beliefs and desires. Let "DB" represent the particular belief/desire complex of some agent and let "R" represent an action brought about exclusively by DB. So, DB causes, but does not determine, R, and it is only DB that is relevant to the occurrence of R (there is no hidden "double" causation). Given that R is an indeterministic consequence of DB, it seems that no one has a choice about whether or not R follows DB. Once DB occurs, given indeterminism, perhaps R will follow and perhaps it will not but since once DB occurs everything relevant to R's occurrence has taken place it seems clear that no one has a choice about R's following DB. Randomness Objection in the standard argument against free will. Similarly, his Consequence argument is the Determinism Objection in the standard argument against free will.