For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some 'organic element' since I couldn't accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
How would our minds not being governed by this universe imply free will?
If anything, I'd assume the you don't have free will if your actions weren't chosen from experience, but were controlled by a supernatural ghost.