The territory that psychologists explore is still largely uncharted; so to find Eysenck’s model for personality in the middle of this terra incognita is rather like stumbling across St. Pancras Station in the heart of the African jungle. Faced with this apparition, one’s first question is, not “does it work?”, but “what’s it for?” This, indeed, is the right question to ask. Eysenck’s model bestrides the field of personality like a colossus. There have been other attempts to describe personality, notably Cattell’s and Guilford’s, and other attempts to explain it, above all, Pavlov’s and Teplov’s: but no one has tried to achieve both these aims on the same scale as Eysenck. In consequence, it is extremely difficult to see the Eysenckian edifice in perspective: there are too few other buildings with which to compare it, only the surrounding trackless jungle. It is by asking “what’s it for?” that we can best provide this perspective. In answer to this question, Fig. 8.1 dis plays what I take to be the general structure of Eysenck’s theory of extra version-introversion (E-I) and neuroticism (N).