"Models of human reasoning are clearly relevant to a wide variety of subject areas such as sociology, economics, psychology, artificial intelligence and man-machine systems. Broadly there are two types: psychological models of what people actually do; and formal models of what logicians and philosophers feel a rational individual would, or should, do. The main problem with the former is that it is extremely difficult to monitor thought processes - the behaviourist approach is perhaps reasonable with rats but a ridiculously inadequate source of data on man - the introspectionist approach is far more successful [e.g. in analysing human chess strategy... but the data obtained is still incomplete and may not reflect the actual thought processes involved."