Abstract
Functionalists define a given mental state as a state that is apt to be the cause of specific effects and the effect of specific causes. Two tokens of the same belief, however, often cause and are caused by very different events: what makes them beliefs of the same type? Several answers, including the one relying on the identity of actual plus counterfactual causal relations, are considered and rejected. Functionalists did not notice that they have to specify how a state which is to be identified as mental is to be individuated, but, given their theory, this cannot be done