 Lewis simply assumes that common sense is resolutely committed to the idea that mental states are characterized by causal role; as a result, his functionalist conclusion drives his characterization of the platitudes. No doubt Lewis has philosophical arguments for denying that mental states are substances that possess their causal powers non-essentially or substances that lack causal powers altogether. But that is beside the present point. Lewis aimed to capture what the folk think about mental states, not what the philosophical literati think.