It seems that we do think about things and adjudicate between good ideas and bad ideas and, therefore, exercise rationality. But all physical systems ultimately are determined. Well, some kind of chemistry in the body. Some kind of physical stimulus to the end of the finger or something like that, which ends up in the feeling of pain, but it's some other thing that caused that. And you see you can keep going back further and further and further, and there's always going to be some prior cause, because all purely physical systems are always the result of some prior necessary and sufficient physical condition that causes the subsequent condition.
Physical systems are always deterministic.
This is why science works. It doesn't effect anything. It's just riding along. And if it doesn't effect anything, then I don't choose. What appear to be choices are merely the results of chemical reactions that happen on their own in a deterministic way in my physical body.