Now that an instinct entirely directed to that which is to be produced lies at the foundation of all sexual love will receive complete confirmation from the fuller analysis of it, which we cannot therefore avoid. First of all we have to remark here that by nature man is inclined to inconstancy in love, woman to constancy. The love of the man sinks perceptibly from the moment it has obtained satisfaction; almost every other woman charms him more than the one he already possesses; he longs for variety. The love of the woman, on the other hand, increases just from that moment. This is a consequence of the aim of nature which is directed to the maintenance, and therefore to the greatest possible increase, of the species. The man can easily beget over a hundred children a year; the woman, on the contrary, with however many men, can yet only bring one child a year into the world (leaving twin births out of account). Therefore the man always looks about after other women; the woman, again, sticks firmly to the one man; for nature moves her, instinctively and without reflection, to retain the nourisher and protector of the future offspring. Accordingly faithfulness in marriage is with the man artificial, with the woman it is natural, and thus adultery on the part of the woman is much less pardonable than on the part of the man, both objectively on account of the consequences and also subjectively on account of its unnaturalness.
But in order to be thorough and gain full conviction that the pleasure in the other sex, however objective it may seem to us, is yet merely disguised instinct, _i.e._, sense of the species, which strives to maintain its type, we must investigate more fully the considerations which guide us in this pleasure, and enter into the details of this, rarely as these details which will have to be mentioned here may have figured in a philosophical work before. These considerations divide themselves into those which directly concern the type of the species, _i.e._, beauty, those which are concerned with physical qualities, and lastly, those which are merely relative, which arise from the requisite correction or neutralisation of the one-sided qualities and abnormities of the two individuals by each other. We shall go through them one by one.