In the first place, a man in love is by nature inclined to be inconstant, while a woman constant.  A man’s love perceptibly decreases after a certain period; almost every other woman charms him more than the one he already possesses; he longs for change:  while a woman’s love increases from the very moment it is returned.  This is because nature aims at the preservation of the species, and consequently at as great an increase in it as possible....  This is why a man is always desiring other women, while a woman always clings to one man; for nature compels her intuitively and unconsciously to take care of the supporter and protector of the future offspring.  For this reason conjugal fidelity is artificial with the man but natural to a woman.  Hence a woman’s infidelity, looked at objectively on account of the consequences, and subjectively on account of its unnaturalness, is much more unpardonable than a man’s.

In order to be quite clear and perfectly convinced that the delight we take in the other sex, however objective it may seem to be, is nevertheless merely instinct disguised, in other words, the sense of the species striving to preserve its type, it will be necessary to investigate more closely the considerations which influence us in this, and go into details, strange as it may seem for these details to figure in a philosophical work.  These considerations may be classed in the following way:—­

Those that immediately concern the type of the species, id est, beauty; those that concern other physical qualities; and finally, those that are merely relative and spring from the necessary correction or neutralisation of the one-sided qualities and abnormities of the two individuals by each other.  Let us look at these considerations separately.

The first consideration that influences our choice and feelings is age....

The second consideration is that of health:  a severe illness may alarm us for the time being, but an illness of a chronic nature or even cachexy frightens us away, because it would be transmitted.

The third consideration is the skeleton, since it is the foundation of the type of the species.  Next to old age and disease, nothing disgusts us so much as a deformed shape; even the most beautiful face cannot make amends for it—­in fact, the ugliest face combined with a well-grown shape is infinitely preferable.  Moreover, we are most keenly sensible of every malformation of the skeleton; as, for instance, a stunted, short-legged form, and the like, or a limping gait when it is not the result of some extraneous accident:  while a conspicuously beautiful figure compensates for every defect.  It delights us.  Further, the great importance which is attached to small feet!  This is because the size of the foot is an essential characteristic of the species, for no animal has the tarsus and metatarsus combined so small as man; hence the uprightness of his gait:  he is a plantigrade.  And Jesus Sirach has said[17] (according to the improved translation by Kraus), “A woman that is well grown and has beautiful feet is like pillars of gold in sockets of silver.”  The teeth, too, are important, because they are essential for nourishment, and quite peculiarly hereditary.