This argument is abductive in character. I am tempted to say that it is unconvincing either because the notion of probability it employs, subjective probability, is irrelevant to the strength of abductive arguments, or because in general no philosophical thesis can be convincingly established through abductive reasoning.
The basic datum which supports it is that indeterminism is extremely surprising. In general, historically speaking, almost everyone who thought about how the universe worked seriously was a determinist, or at least thought that every event had a cause, a pretty closely related view.
And pretty much the only reason anyone who thought about it wasn't a determinist was because they believed in libertarian free will. It was a conviction that libertarian free will existed and that at least some indeterminism in the universe was necessary for libertarian free will that was the primary historical driver for belief in some form of indeterminism.
Well, indeterminism is true, as far as we know: quantum mechanics still carries the day in fundamental physics. This theory was and remains surprising to many people's philosophical intuitions. But it is established fact as much as anything in science is - more than most things.
The idea, then, is this: indeterminism was overwhelmingly surprising unless one believed in free will; the subjective probability of determinism approached a certainty when the world was considered from any perspective other than that of our supposed human freedom. Thus we can set up the usual abductive schema:
1. If the free will hypothesis is true, the universe is at least partly indeterministic.
2. If the free will hypothesis is not true, the universe is highly likely to be deterministic.
3. The universe is at least partly indeterministic.
C. Therefore, the free will hypothesis is very likely to be true.
(2) is of course not a statement of logical connection between the absence of free will and determinism, although if incompatibilism is true than the reverse entailment holds. Rather it is an inductive thesis concerning the subjective likelihood of determinism being true reached by an examination of as much pre-20th century philosophical and scientific thought as we care to undertake: smart people who thought about it were generally determinists on the basis of observation and/or thought unless a belief in libertarian free will led them to be otherwise. This thesis might be wrong - I can hardlly claim to have made a systematic study - but for what it is worth I think it is right.
As abductive arguments go in philosophy I consider this better than some. I also consider it totally unpersuasive, but I'm not completely sure why. One might be the contingency of premises (2) and (3) - even if they were right, and indeed even if humans were smart enough that our subjective likelihoods for the truth or falsehood of various metaphysical hypotheses correlated with their actual likelihoods, if there are any, and so we had some genuine reason to give credence to metaphysical hypotheses widely believed by intelligent people, I still would not find an argument for free will based on such premises very useful. What if new science comes in showing that quantum mechanics is false, or that our forebears had systematic cognitive illusions that made them grossly exaggerate the likelihood of a deterministic universe? Well, what if. This might be a matter of taste, but there is a taste according to which the dependence of an argument upon such facts, revisable in the light of new experience, makes it unsuitable for establishing any metaphysical thesis.
Another reason it might be unpersuasive is a sort of Olympian viewpoint with respect to (2), on which the opinions of even the most brilliant and learned count for nothing relative to the facts themselves. This would be a prejudice according to which you can never argue from what people believe or believed about any given thesis to the truth or falsehood of that thesis.
I posted this mostly because I thought the line of reasoning was novel - I hadn't seen it before at any rate. Doesn't mean it's good.
I have a more serious argument for libertarian free will which I am going to try to compose into a paper. I will post the results of that research here if the research fails, or a link to the paper if it succeeds.
Happy new year!
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