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.
The fun thing is, even if we assume our consciousness isn’t entirely deterministic, the most reasonable alternative would be pure randomness.
Which, in the end, makes absolutely no difference in the free will argument.