Psychologists have for many years tried to grapple with the problems of dealing “scientifically” with the psychology of thinking. These attempts have met with little success, in the sense that no agreement has been reached on the definition of what thinking is and how it differs from other forms of behaviour, nor has any satisfactory explanation of thinking been produced. Indeed, one may well ask: what would constitute a satisfactory explanation of thinking ? This is a question which has not generally bothered psychologists particularly, although attempts at explanation suffer changes of fashion from time to time.