Why mental states are not physical states

If mental states are physical states, then the truth about what someone believes should follow necessarily from the state of the brain/physical world. But it doesn't. If we line up all the physical facts, we have every atom traced, the argument from The brain is in state X therefore he must believe, say, that God exists, cannot follow necessarily. The state of the physical always leaves the state of the mental indeterminate. But what my thought is about is determinate, not indeterminate. Therefore my belief is not a physical state.