It is often said (for example, Kendler, 1970) that psychology has three subject matters: conscious experience, behaviour and physiology. The mind-body problem is concerned with the relation between two of these; namely, consciousness and neurophysiological processes. Part of the puzzle is how the latter can give rise to the former when they appear to be so different in kind. As they stand, the concepts 'mind' and 'body' are probably too vague to be useful. 'Mind' is commonly used in two main senses: (1) conscious experience, and (2) the system or program that governs behaviour. Strictly speaking, the mind-body problem is concerned with the former. It is, however, sometimes discussed in terms of the latter (for instance, Fodor, 1981), where 'mind' refers to the software and 'body' refers to the hardware. Mental phenomena include sensations, images, feelings, thoughts, beliefs, intentions and decisions. 'Body' appears to be more straightforward and refers to the physical aspect of an organism, in this case particularly the brain. Described by Schopenhauer as 'the world knot', the problem, like so many others treated in this book, is a tangle of conceptual and empirical issues; both logical arguments and scientific discoveries are relevant.