The proportion of the male to the female principle in the same human being must not be assumed to be a constant quantity. An important new conclusion must be taken into account, a conclusion which is necessary to the right application of the principle which clears up in a striking fashion earlier psychological work. The fact is that every human being varies or oscillates between the maleness and the femaleness of his constitution. In some cases these oscillations are abnormally large, in other cases so small as to escape observation, but they are always present, and when they are great they may even reveal themselves in the outward aspect of the body. Like the variations in the magnetism of the earth, these sexual oscillations are either regular or irregular. The regular forms are sometimes minute; for instance, many men feel more male at night. The large and regular oscillations correspond to the great divisions of organic life to which attention is only now being directed, and they may throw light upon many puzzling phenomena. The irregular oscillations probably depend chiefly upon the environment, as for instance on the sexuality of surrounding human beings. They may help to explain some curious points in the psychology of a crowd which have not yet received sufficient attention. In short, bi-sexuality cannot be properly observed in a single moment, but must be studied through successive periods of time. This time-element in psychological differences of sexuality may be regularly periodic or not. The swing towards one pole of sexuality may be greater than the following swing to the other side. Although theoretically possible, it seems to be extremely rare for the swing to the male side to be exactly equal to the swing towards the female side. It may be admitted in principle, before proceeding to detailed investigation, that the conception of sexually intermediate forms makes possible a more accurate description of individual characters in so far as it aids in determining the proportion of male and female in each individual, and of measuring the oscillations to each side of which any individual is capable. A point of method must be decided at once, as upon it depends the course the investigation will pursue. Are we to begin by an empirical investigation of the almost innumerable intermediate conditions in mental sexuality, or are we to set out with the abstract sexual types, the ideal psychological man and woman, and then investigate deductively how far such ideal pictures correspond with concrete cases? The former method is that which the development of psychological knowledge has pursued; ideals have been derived from facts, sexual types constructed from observation of the manifold complexity of nature; it would be inductive and analytic. The latter mode, deductive and synthetic, is more in accordance with formal logic.