Alternative possibilities for Free Will depend on the reality, the metaphysical possibility, of possibilities. Actualists deny them.
Historically, the opposition to metaphysical possibility has come from those who claim that the only possible things that can happen are the actual things that do happen. To say that things could have been otherwise is a mistake, say eliminative materialists and determinists. Those other possibilities simply never existed in the past. The only possible past is the past we have actually had.
Similarly, there is only one possible future. Whatever will happen, will happen. The idea that many different things can happen, the reality of modality and words like “may” or “might” are used in everyday conversation, but they have no place in metaphysical reality. The only “actual” events or things are what exists. For “presentists,” even the past does not exist. Everything we remember about past events is just a set of “Ideas.” And philosophers have always been troubled about the ontological status of Plato’s abstract “Forms,” entities like the numbers, geometric figures, mythical beasts, and other fictions.
Traditionally, those who deny possibilities in this way have been called “Actualists.”
In the last half-century, one might think that metaphysical possibilities have been restored with the development of modal logic. So-called modal operators like “necessarily” and “possibly” have been added to the structurally similar quantification operators “for all” and “for some.” The metaphysical literature is full of talk about “possible worlds.”
The most popular theory of “possible worlds” is David Lewis‘s “modal realism,” an infinite number of worlds , each of which is just as actual (eliminative materialist and determinist) for its inhabitants as our world.
It comes as a shock to learn that every “possible world” is just as actual, for its inhabitants, as our world is for us. There are no alternative possibilities, no contingency, that things might have been otherwise, in any of these possible worlds. Every world is as physically deterministic as our own.