1. That there is nothing in reality other than the physical objects studied by science
2. That the movement of these physical objects are governed by fixed mathematical laws.
Putting these two things together, we get determinism. This is the idea that humans don’t actually have free will, but simply do the things that the ongoing electro-chemical reactions in our brains direct us to do. Of course, there is one very big problem with believing determinism is true.
It can’t be rationally believed.
To say that one’s thoughts and actions are being directed, not by one’s will, but by physical processes in the brain is to undercut any reason one has for believing anything one thinks. It is the declaration that one believes in determinism, not because it is true but simply because one was determined to believe it. In fact, it is claiming that everything one believes has this problem.
So, to say that determinism is true is to claim that one’s beliefs are based in chemical reactions, not rational consideration.
They may line up with reality, of course. People may be “wired” to get our bodies where they need to be in the physical world, in the same sense that a calculator is wired to spit back the correct answer to an equation, but this doesn’t mean that we can ever chose to be rational people. Rather we simply believe what we are “programmed” to believe.
Strange, then, that so many believers in determinism complain that others aren’t more open-minded, or don’t think critically. By their own admission, they themselves can’t be open-minded to anything except what certain physical laws have determined them to believe.
There is a list of objections to determinism, of course. But I mostly find myself wondering why, if the philosophy leads us to the point that we’re rejecting our own ability to think rationally, we don’t want to reconsider the assumptions that got us here.
But that leaves us rejecting one or the other of the two ideas stated above. Any who are convinced of the second would need to reject the first of them–which is precisely what the theist does.