to say “could not have done otherwise” is a modal fallacy. Given certain antecedents, I WILL, but not MUST, do a certain thing.
Incompatibilist-determinists can acknowledge the must/will distinction and assert the "WILL not have done otherwise", and their viewpoint remains viable. Frankly, I think it remains essentially unchanged, but that is for them to say.
In (let's call it) a world of modal logic, in a reality for which modal logic is the most apt way of describing (at least aspects of) the actual world, there are actual (meaning concurrently available alternative) possibilities, and there might be necessities. Actual possibilities are matters of indeterminateness. If the modal logic world were only a world of determinateness, then there would not be the non-determinateness, the not-determined-ness, i.e., the indeterminateness which provides for, and is expressible in terms of, possibilities - such as those which (maybe just some) humans think they discern at the macrophysical level.
Now, the incompatibilist-determinists who hold to physicalism (those appearing to be the sort of determinists who have been involved in this discussion) seem to assert that physics is somehow sufficient to preclude the indeterminateness and the possibilities that are to be found in the modal logic world. These incompatibilist-determinists can even acknowledge that there is quantum indeterminateness and dismiss that sort of indeterminateness as irrelevant with regards to the relatively macrophysical occurrences discussed, for instance, in terms of human acts. After all, humans do not resort to quantum-level details when describing human acts.
For these determinists, there is no indeterminateness subject to human action for a human-controlled conversion to determinateness. These determinists agree with
the notion that no "imaginary indeterminateness ... is needed" in order to explain the world or what humans do. These determinists deny the actuality of the indeterminateness and possibilities which can be found in the modal logic world. In essence, these determinists hold that the modal logic world is imaginary, not actual.
In that case, the logic found in physics (or, more specifically, macrophysics) is held to be the only apt logic with regards to human matters, and that is a logic without possibilities (assuming, of course, that there is logic if there are no possibilities). As a logic without possibilities, it is also a logic of inexorability without modal necessity. What will be will be, and it is never not determined what humans do to effect what will be - even though humans do not perceive (and, therefore, feel free from) the inexorability of the logical process that is physicalist (or scientistic) physics.
If the foregoing well enough captures the incompatibilist-determinist position, with what do the compatibilist-determinists disagree?
