A pattern of argument against determinism that is worth exploring is not to argue that free will is incompatible with a variety of determinism, but instead to argue that some other feature of our moral life is incompatible that variety of determinism. For instance, the following argument is valid:
- If theistic determinism holds, then all our character features, histories and choices are entirely determined by God. (Premise)
- If x's character features, history and the choice to promise p to y, are entirely determined by y, then x's promise of p to y is invalid (and hence non-binding). (Premise)
- One only validly promises something if one chose to promise it. (Premise)
- Therefore, if theistic determinism holds, no promises to God are valid (by 1-3).
- Some promises to God are valid. (Premise)
- Therefore, theistic determinism is false. (By 4 and 5)
I think one can use (2) in an argument for incompatibilism. You start with (2) and add the premises:
- If x freely promises to y something permissible, then x's promise is valid.
- If some free choice can have an ultimate cause outside one, then a freely made promise of something permissible to any person whatsoever can have an ultimate cause outside one.
- Whether an action is free does not depend on conditions a hundred years before one's conception.