Jung wrote his Type Psychology in 1914, but does not seem to have developed any further its general thesis. The terms “introvert” and “extravert”, however, have slipped into everyday language, and experimental studies and questionnaire inquiries on introversion-extraversion continue to be published in psychological journals. Ink-blots, ambiguous figures, and even knee-jerks are called upon to measure this fundamental tendency of man; inventories and self-rating sheets are employed widely to estimate it; and yet it appears to evade capture. There is as yet little or no evidence anywhere that a coherent tendency of the kind looked for does in fact exist.