I have an argument regarding the validity of rationalism in a universe without non-deterministic free will.
If human being's theories, thoughts and ultimate conclusions are inevitable, (bound by determinism), then isn't it logically invalid for us to assign the term "rational" or "true" to any of our conclusions being that they were inevitable regardless of their rationality or truth. There would be no test that I would have been free to perform that could have allowed me to distinguish which of my inevitable conclusions were "true" or "not "true" or "rational" or merely "apparently rational".
It seems to me that inability to have believed otherwise is the highest description that can be made of a belief or conclusion, and that adding the un-testable term "rational" or "true" on top of the term "'inevitable" is logically invalid.
This would suggest that the term "rational belief" is invalid in a real world where all beliefs were inevitable.
We could possibly extend the argument I have made to say that if one person believes that they lack free will, then they can never rationally prove that anyone else lacks free will because the first person would have to admit that they may be incapable of having a belief that could have been otherwise, even if it was false.