I’m a little uncomfortable reading Wallace’s book since it was a youthful work not intended for publication, was never published while he lived, and is being published now without his permission. And he left no later comments on it and can’t respond to critics.
Wallace’s solution depends on a scope difference in an alethic tensed modality. Using Taylor’s example: “if the battle occurred, then the admiral must have ordered it” is ambiguous between
1. if the battle occurred, then yesterday it was the case that the admiral must have ordered it
2. if the battle occurred, then it must have been the case that the admiral ordered it
Wallace admits that (1) entails fatalism, but points out that (2) doesn’t. According to (1), the admiral must have ordered the battle, and so had no choice. In (2) the admiral ordered it, but not under duress, as it were, of necessity (must). He had a choice — he might have contemplated several possible worlds in which he orders and several in which he declines to order. It’s just that none of those possible worlds turn out to have been real. That is, yesterday’s world in which the battle was ordered, turns out to have been the only possible world.
But if that’s the only possible world, why is it the only? Wallace seems to show successfully that the answer cannot be the logic alone.
Suppose you are at the moment of ordering. That moment excludes any moment in which you decline to order. That moment includes only moments in which you order the battle. The difference seems to be between whether you have free will or whether you have freedom. Wallace’s draws a nice distinction between fatalism and a kind of post hoc determinism.
Is this a difference without a distinction? If the admiral knows that the moment determines his order (he has no freedom), what does it serve him to have free will? Nothing in the real world. But that accords with our experience: no matter how we plan for the world, the consequences are beyond our ability to control.
The utilitarian/consequentialist effects of determinism and fatalism are equally discouraging. But the entailments for (Kantian) moral sensibility are completely distinct. Determinism is consistent with holding moral sentiments. Not a fatalist, and that’s why even philosophers spurn it.
On the other hand, while Wallace has found a distinction, I’m not sure that it is telling against Taylor’s view. Relations of necessity among physical effects depend on circumstances, and these are explicit in Taylor’s assumptions. If there is no world in which the order for battle is not given, then the only possible worlds are those in which he chooses to order it. That entails a strange paradox: he is free to choose, but he can only choose one option, not the alternate choice.
How the logic of implication works entirely depends on how you set up the modal system — its axiomata or its inferential rules or both — and its consistency. What makes a system meaningful, assuming it’s consistent, is its usefulness or accuracy. Wallace uses our natural language notions of “it couldn’t have happened” and “it can’t have happened.” That’s good for his system, but not telling against Taylor, since Taylor is specifically using logic against natural language notions which, he is attempting to show, are wrong. And on the other hand, Wallace’s distinction seems to violate our linguistic, and maybe real-world, understanding of “free.” It may be that the logical syntax should include an inference from
must yesterday order
yesterday must order
or it may be that the inference should be dealt with in the semantics, in the model — in any world in which there is only one option, there is no free choice.