The author contends that a functionalist methodology in psychology and neurophysiology will prove adequate to explain human behaviour. Defence of this thesis requires: an examination of the mental/physical dichotomy, and its rejection in favour of a distinction between psychological and physical terms; a description and discussion of functionalism in psychology and neurophysiology, showing how the notorious problem of the necessary intensionality of psychological terms may be circumvented; an examination of the role of computer simulation in psycho-physical research; and an explanation of how the phenomena of sentience fit the functional framework. The book concludes that the thesis presented is in all essentials that of Aristotle; Aristotle had no ‘mind-body problem’, and were it not for a subsequent over-obsession with Cartesian scepticism, we need not have had one either.