A long time ago, everyone thought bachelors were unmarried and that it was wrong for a married man, but not a bachelor, to have sex with multiple people.  Then a philosopher got the bright idea of redefining “bachelor” to include “those married men who didn’t have much sex with their wives.”  That redefinition allowed some married men to think of themselves as free to pursue other women. They were happier.

Although it’s a stretch, something like this seems to me to parallel what has happened with the rise of “compatibilism.” That is the notion that although the universe may be deterministic in a physical way—so that our actions and thoughts are not only determined by the laws of physics, but also predictable if we had enough foreknowledge—we nevertheless have “free will.”  What philosophers did was redefine the meaning of “free will” away from its historical and religious sense, so that “free”, instead of meaning “independent of the strictures of your bodily makeup and environmental influences”, now meant a variety of other things, like, “your decision isn’t being made with a gun to your head.”

There is no one form of compatibilism: various philosophers have suggested various tweaks that allow us to say we have “free will.”

To me, the important aspect of this debate came not from philosophy but from science: we realized that our brains, like all physical objects, are subject to the laws of physics, and there was no way that some nonmaterial spook in one’s head could make “free decisions”. That was something new.