What Kind of Modality Does the Materialist Need for His Supervenience Claim?

Materialists who do not deny the existence of mental phenomena usually claim that the mental supervenes on the physical, i.e. that there cannot be a change in the mental life of a man without there being a change in the man's body. This modal claim is usually understood in terms of logical necessity. I argue that this is a mistake, resulting from assumptions inherited from logical empiricism, and that it should be understood in terms of synthetic necessity.