This paper takes as its starting point that human beings do experience pain, and that there is a correlation between incidents of pain and tissue damage. Either this trait is present in the human phenotype because it was selected for or its presence is a byproduct of selection for some other trait. In order for there to be selection for a trait, that trait must make a difference to the organism possessing it: pain states must be causal states if they are to be selectively advantageous. Pain accompanies too wide a range of physical phenomena to be a byproduct of selection for something else. Thus pain states are causal states, and some mental states (at least) are causally efficacious.