Epiphenomenalism is the belief that mental states cannot cause any physical events. It is necessarily a determinist view. The identity theory is the theory that mental states are identical with brain states. An identity theorist believes that all brain states are reducible to brain states. So when we think we are thirsty, what is really going on is our brain has received signals from other parts of our body that are sent when there is not enough water in the system. Thirst is then reducible to a brain state. Likewise, when we think we are making a decision to lift our arm, what is really going on is some brain process, whatever that will turn out to be. A decision, just like any other mental state, is really just a brain state. So whatever brain state causes the brain state that appears to be our decision to raise an arm, that is the real cause of the arm going up. The mental state didn’t cause anything, it was a brain state—what the mental state appears to be—that caused the physical event. That is exactly the deterministic view of epiphenomenalism.