It is not surprising that the characteristically sexual method of display and emotional appeal should be associated with the earlier efforts at adjustment, both in the individual and in the state. This method is based on the instincts, and just as inhibition and brain legislation follow the instincts in point of development, a rational mode of control, individual and public, is developed later than the emotional form, or, at any rate, is not at first independent of it.
The origin of mental impressionability seems to lie then, not in one, but in the two general regions of activity—that connected with the struggle for food and that connected with reproduction. The strain on the attention in the food and conflict side of life involves the development of mental impressionability, particularly of an impressionability on the side of cognition. But in addition we have the impressionability growing out of sexual life which has been in question above, and which is more closely related to appreciation than to cognition. And of these two aspects of impressionability—the one growing out of conflict and the one growing out of reproduction—the latter has more social possibilities than the former, because it implies a sympathetic rather than an antagonistic organic attitude. It is certainly in virtue of susceptibility to the opinion of others that society works—through public opinion, fashion, tradition, reproof, encouragement, precept, and doctrine—to bring the individual under control and make him a member of society; and it is doubtful whether this could have been accomplished if a peculiar attitude of responsiveness to opinion had not arisen in sexual relations, reinforcing the more general and cognitive impressionability. Without this capacity to be influenced the individual would be in the condition of the hardened criminal, and society would be impossible.
This sex-susceptibility, which was originally developed as an accessory of reproduction and had no social meaning whatever, has thus, in the struggle of society to obtain a hold on the individual, become a social factor of great importance, and together with another product of sexual life—the love of offspring—it is, I suspect, the most immediate source of our sympathetic attitudes in general, and an important force in the development of the ideal, moral, and aesthetic sides of life.