The building of a science is in many ways like the building of a cathedral or the building of a city. It is rarely, if ever, a matter of starting on a virgin site and of executing in detail a preconceived plan. More often than not it is a matter of continuous alteration, of repeated destruction and reconstruction. At any time the edifice will display the styles of many ages. This is certainly so with Psychology. The science so described is an edifice which today has some of the features of the Parthenon, and some of the features of the new Coventry Cathedral. If its original architect could be named it would be A ristotle, but the system incorporates elements deriving from the pre-S ocratics and elements from primitive thought; and like other ancient foundations it embodies much picturesque but functionless ornamentation. It follows that a good psychologist must be a man of several parts. He must be original and creative, but he must also be critical and destructive. The needed qualities of mind are rarely combined, and in general the progress of psychology depends upon the kind of cooperation that consists in conflict. Bold creative thinkers (as we describe them when they have our approval) or armchair speculators (as we describe them when they have not) submit their creative thoughts to be manhandled and mauled by carping destructive critics and by the manipulators of slide rules. Never does a grand overall theory survive in the form in which it was originally conceived. Rarely, if ever, is it utterly destroyed. Something comes out of this conflict which neither the creative thinker nor the destructive critic could have produced alone. Sometimes the qualities of the destructive critic are combined with those of the constructive thinker -as in the case of the author of this book. Dr Eysenck can manipulate slide rules, but he can also manipulate concepts.