Determinism nowadays must reach outside of Newton's laws precisely because they are insufficient to account for all phenomena. Nevertheless, he had never come up with those laws had he not observed patterns in the world around him and we would never have improved upon those laws were it not for more accurate measurements ever since. The entire enterprise of science in general and physics in particular is possible only under the tentative assumption that the reality we find ourselves in, as a matter of fact, as an actual reality operates on actual and (perhaps conveniently) intelligible rules, and same enterprise couldn't possibly be anywhere near as successful as it is, if that assumption was severely incorrect.
So with and without our access to perfectly accurate descriptions of any one, much less all phenomena of reality, there is a good chance that reality as a whole and in parts is less than capricious and random and arbitrary. It may not be a perfectly safe bet, but to put your figuritive money on the contrary is foolish, to put it kindly. Besides, if some truly random force of nature truly randomly moved you to a decision, as ren pointed out, where exactly was your free will anyway? Even if all of our brain chemistry was completely unpredictable, free will would still remain an open question, arguably even more so than it is otherwise, as ren pointed out.
For all we know determinism is more likely true than not.
So what? How is anything different given this realization?
That's what I meant when I said that it is similar to hard solipsism. It is not an illegitimate thought at all. It just happens to be an intellectual dead end that neither leads to anything beyond itself nor helps with anything you passed on your way there. Unlike solipsism, determinism keeps gathering evidence in its favour. But it's still a dead end.
Better to leave questions unanswered than answers unquestioned
