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Quite early in my practice of psychoanalysis I commenced to observe certain marked lines of difference in the psychology of my patients, and it was not long before they appeared to fall into distinct classes with well-defined outlines. Just at this time I received Jung's little paper called “A Preliminary Study of Psychological Types,”1 which at once opened a door throwing a flood of light on the matter, and proved a great stimulus to my own observations. Now I recognized that I had been separating the extraverts and the introverts as Jung designated them, and could clearly distinguish and agree with most of his findings. In his paper Jung referred to only two types which he designated introvert and extravert according to the primary movement of the libido2 toward the center, the ego, or toward the periphery, the object. He suggested that there may be others still undefined, but confined his discussion to the most obvious characteristics of these two main divisions.3
As time went on, however, it became increasingly evident that finer differentiation was necessary if any deeper understanding of personality was to be gained. For, while these two major directions of the libido were everywhere obvious, other elements entered into