Sec. 13. Half-way between vital and social functions, lies the sexual instinct. If nature had solved the problem of reproduction without the differentiation of sex, our emotional life would have been radically different. So profound and, especially in woman, so pervasive an influence does this function exert, that we should betray an entirely unreal view of human nature if we did not inquire into the relations of sex with our aesthetic susceptibility. We must not expect, however, any great difference between man and woman in the scope or objects of aesthetic interest: what is important in emotional life is not which sex an animal has, but that it has sex at all. For if we consider the difficult problem which nature had to solve in sexual reproduction, and the nice adjustment of instinct which it demands, we shall see that the reactions and susceptibilities which must be implanted in the individual are for the most part identical in both sexes, as the sexual organization is itself fundamentally similar in both. Indeed, individuals of various species and the whole animal kingdom have the same sexual disposition, although, of course, the particular object destined to call forth the complete sexual reaction, differs with every species, and with each sex. If we were dealing with the philosophy of love, and not with that of beauty, our problem would be to find out by what machinery this fundamental susceptibility, common to all animals of both sexes, is gradually directed to more and more definite objects: first, to one species and one sex, and ultimately to one individual. It is not enough that sexual organs should be differentiated: the connexion must be established between them and the outer senses, so that the animal may recognize and pursue the proper object. The case of lifelong fidelity to one mate -- perhaps even to an unsatisfied and hopeless love -- is the maximum of differentiation, which even overleaps the utility which gave it a foothold in nature, and defeats its own object. For the differentiation of the instinct in respect to sex, age, and species is obviously necessary to its success as a device for reproduction. While this differentiation is not complete, -- and it often is not, -- there is a great deal of groping and waste; and the force and constancy of the instinct must make up for its lack of precision. A great deal of vital energy is thus absorbed by this ill-adjusted function. The most economical arrangement which can be conceived, would be one by which only the one female best fitted to bear offspring to a male should arouse his desire, and only so many times as it was well she should grow pregnant, thus leaving his energy and attention free at all other times to exercise the other faculties of his nature.