f the more tender-hearted that
passionate love is sometimes associated with a feeling of quite another
kind--namely, real friendship founded on harmony of sentiment, but this,
however, does not exist until the instinct of sex has been extinguished.
This friendship will generally spring from the fact that the physical,
moral, and intellectual qualities which correspond to and supplement
each other in two individuals in love, in respect of the child to be
born, will also supplement each other in respect of the individuals
themselves as opposite qualities of temperament and intellectual
excellence, and thereby establish a harmony of sentiment.

The whole metaphysics of love which has been treated here is closely
related to my metaphysics in general, and the light it throws upon this
may be said to be as follows.

We have seen that a man's careful choice, developing through innumerable
degrees to passionate love, for the satisfaction of his instinct of sex,
is based upon the fundamental interest he takes in the constitution of
the next generation. This overwhelming interest that he takes verifies
two truths which have been already demonstrated.

First: Man's immortality, which is perpetuated in the future race. For
this interest of so active and zealous a nature, which is neither the
result of reflection nor intention, springs from the innermost
characteristics and tendencies of our being, could not exist so
continuously or exercise such great power over man if the latter were
really transitory and if a race really and totally different to himself
succeeded him merely in point of time.

Second: That his real nature is more closely allied to the species than
to the individual. For this interest that he takes in the special nature
of the species, which is the source of all love, from the most fleeting
emotion to the most serious passion, is in reality the most important
affair in each man's life, the successful or unsuccessful issue of which
touches him more nearly than anything else. This is why it has been
pre-eminently called the "affair of the heart." Everything that merely
concerns one's own person is set aside and sacrificed, if the case
require it, to this interest when it is of a strong and decided nature.
Therefore in this way man proves that he is more interested in the
species than in the individual, and that he lives more directly in the
interest of the species than in that of the individual.

Why, then, is a