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 best theories are non-deterministic, but of course we don’t know if they are the last word about reality. To put it another way, we don’t know why the math is non-deterministic in our best equations.
The old equations were deterministic, but they turned out to be wrong. Something similar may happen here.