Take something more mundane: today it is true that tomorrow, I will stay at home.
- Today it is true that tomorrow I will stay at home. Call that truth S.
- S is true, and permanently true (true at all times).
- Is not possible that both (2) and that I render S false through some action of my own.
- It is not possible that I render S false by some action of my own (follows from 2 and 3).
- I can’t render S false through my actions; therefore, inevitablism is true and tomorrow I must stay at home (I have no choice in the matter).
The above argument is unsound. It commits the modal fallacy at Step 4.
Step 4, when derived from Step 3, looks like this:
I am at home; it is not possible that (I am at home and not at home); therefore, it is not possible that I am not at home.
Perhaps you will see the problem. Of course it’s possible that I’m not at home; it’s just that I AM at home. Nothing about permanent truth (true at all times) entails that I be at home, or that Biden be elected, or anything else. What IS necessary is that no proposition can be both TRUE, and FALSE, at the same time (Law of Noncontradiction).
Put another way, the fallacy occurs thus:
If today it is true that tomorrow I am at home, then tomorrow I MUST (necessarily) be at home.
The modal fallacy lies in ascribing the modal necessity operator to the consequent of the antecedent, thereby illicitly rendering a contingent (could have been otherwise) truth as necessary. The repair goes as follows:
Necessarily (if it is true today that tomorrow I am at home, then tomorrow I WILL BE [not MUST be!] at home).
When you speak, as you so often do, of causal necessity, or physical necessity, or deterministic necessity, or deterministic inevitability, you are committing every single time a modal fallacy because you are confusing absolute necessity with relative necessity. The relative necessity lies in the conjunction of the antecedent and the consequent, rendered just above. But all the above is saying is that necessarily, if a proposition is true, then it is true; not that it MUST be true. If I DON’T stay at home, then the proposition “tomorrow I will be at home” remains truth-valued, only it now returns the value FALSE instead of TRUE.
Moreover, there is no modal category called “causal necessity” or “deterministic inevitability,” or anything of the kind. There is only logical necessity, which deals with true propositions that can’t be false on pain of logical contradiction, like married bachelors or four-sided triangles.
Hence there is no physical or causal or deterministic necessity. We perceive that gravity operates everywhere and in the same way and at all times; yet this is not a necessary truth, because I can easily conceive a possible (though non-actual) world in which gravity does not exist, and I can do so without bringing about a logical contradiction.
Turning to causal determinism, let S stand for “chooses Pepsi” at some time T when I have a choice between Pepsi and Coke.
Your argument until now has been that, given deterministic antecedents x, y, and z, then necessarily S.
But S isn’t necessary, as I have just demonstrated! What you can say is,
Necessarily (given x, y, z, then S), where S is, was, and always will be, contingent (could have been otherwise).
This means that at Time T, not-S was within my power; and more, it is, was, and always will be, true, that not-S is possible. Sure enough, I WILL choose S, given x, y, and z, but I don’t have to.
If I don’t choose S, then different antecedents would be in play.
But, you will object, the antecedents x, y, and z, were inevitable, given determinism, and therefore S is inevitable, too, but that’s wrong — they were not inevitable — for all the reasons given above!
Even IF I always choose S, it follows that it was always within my power to choose not-S.
So when Jerry Coyne, for example, always states “you could not have done, other than what you did,” he is talking logical nonsense. And so are you, if you keep sticking to that line.
A final point about inevitability. One should not confuse “inevitability” with “fixity.” The past is fixed, but it was never inevitable, for reasons outlined above. It’s true that I cannot change the past. But no one I know argues, “Because I can’t change the past, I have no free will.”
Now suppose the future is as fixed as the past, which it is under the block universe model of Minkowski/Einstein. In that case I can’t change the future either, because of Step 3 in the initial argument offered above: Is not possible that both (2) and that I render S false through some action of my own.
But, compatibilist free will (or libertarian, for that matter) does not require changing the past, or the future, or even the present — to do so would entail the following absurdity, that I AM both at home, and not at home, at the same time.
Free will entails only the ability to make, in some small measure, the past, present, and future, be what it was, what it is, and what it will be. This just means that if the future is as fixed as the past, then some of the future is fixed by my actions, just like the present and the past.