Theories about the structure of personality go back at least as far as the ancient Greeks, and the theory of the four humours and the four temperaments corresponding to them is of very respectable antiquity. In its most widely accepted form, it is due to the Greek physician Galen who lived in the second century of our era. The famous German philosopher Immanuel Kant accepted this theory in his widely read textbook of psychology which he entitled Anthropologie, and his descriptions of the four temperaments have become widely quoted. He followed custom in conceiving of them as being separate categories into one or other of which every person could be sorted, without any possibility of overlap or change (Eysenck, 1960e). This was clearly not in accord with reality, and towards the end of the nineteenth century various psychologists pointed out that a better description of personality could be achieved by using two orthogonal dimensions, along each of which people could be distributed continuously. Wilhelm Wundt postulated that one of these dimensions would segregate the strong from the weak emotions, i.e., the melancholic and choleric temperaments from the phlegmatic and sanguine; as his other dimension he postulated one which divorced the changeable temperaments, the choleric and sanguine, from the unchangeable ones, the melancholic and the phlegmatic (Eysenck and Eysenck, 1967). Figure 13 shows in diagrammatic form this combination of the theories of Kant and Wundt. Traits traditionally describing the “four temperaments” of Galen. The actual descriptions are adapted from I. Kant, and the arrangement about the two major axes is as suggested by W. Wundt. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781351305280/f4a13897-1fb0-4f23-a781-7f7969512828/content/fig13_OB.tif"/>