Free Will A Contemporary Introduction

Free Will, Moral Responsibility, Determinism 21

issues. Here we will be interested primarily in a mistake in reasoning about
modality, a kind of modal fallacy. We will only explain this informally, since
this will be enough to serve our purposes. Consider this non- modal pattern of
inference:

Jasmine is a pediatrician.

If someone is a pediatrician, then she is a physician.

Therefore, Jasmine is a physician.

Now consider a modal correlate of this pattern:

Jasmine is a pediatrician.

Necessarily, if someone is a pediatrician, then she is a physician.

Therefore, necessarily, Jasmine is a physician.

This modal inference is fallacious. It is true, we’re supposing, that Jasmine is a
pediatrician. But from this we cannot conclude that it’s necessary that she’s a
pediatrician; we can assume only that it’s a contingent truth. But there is a neces-
sary conditional relation between being a pediatrician and being a physician: it’s
impossible to be a pediatrician without being a physician. But it’s clearly not
necessary that Jasmine is a physician. So the premises of the argument, on our
supposition, are true, but the conclusion is false, and so the argument is unsound.
The diagnosis is that it is a modal fallacy to transfer the necessity of the condi-
tional in the second premise to the consequent of that conditional unless the
antecedent is also necessarily true. We can infer from the premises that Jasmine
is a physician, but not the claim that, necessarily, she is physician.
Return now to the claim that it follows from the truth of determinism that it is
metaphysically necessary that Dana took her kids to school. Inferring this is fal-
lacious in just the sense identified above. Again, treat “P” as a proposition
expressing the entire state of the universe at some instant in the past, and treat
“L” as a proposition expressing the entirety of the laws of nature. Now, if deter-
minism is true, then, necessarily, it is a consequence of the facts of the past, P,
and the laws of nature, L, that every truth obtained about the entire state of the
universe during the duration of time it took for Dana to drive her kids to school
this morning. So, the truth of Dana’s driving her kids to school this morning is
entailed by the conjunction of P and L. Here now is the fallacious reasoning that
led to the illicit conclusion:

P and L.

Necessarily, if P and L, then Dana drove her kids to school this morning.

Therefore, necessarily, Dana drove her kids to school this morning.

All that we can infer from the two premises is that Dana drove her kids to school
this morning; we cannot infer that it was necessary that she did so. We can claim
that under the assumption that determinism is true, necessarily, Dana drove her