It has been seen that the intensity of love grows with its individuation; we have shown that two individuals may be so physically constituted, that, in order to restore the best possible type of the species, the one is the special and perfect complement of the other, which, in consequence, exclusively desires it.  In a case of this kind, passionate love arises, and as it is bestowed on one object, and one only—­that is to say, because it appears in the special service of the species—­it immediately assumes a nobler and sublimer nature.  On the other hand, mere sexual instinct is base, because, without individuation, it is directed to all, and strives to preserve the species merely as regards quantity with little regard for quality.  Intense love concentrated on one individual may develop to such a degree, that unless it is gratified all the good things of this world, and even life itself, lose their importance.  It then becomes a desire, the intensity of which is like none other; consequently it will make any kind of sacrifice, and should it happen that it cannot be gratified, it may lead to madness or even suicide.  Besides these unconscious considerations which are the source of passionate love, there must be still others, which we have not so directly before us.  Therefore, we must take it for granted that here there is not only a fitness of constitution but also a special fitness between the man’s will and the woman’s intellect, in consequence of which a perfectly definite individual can be born to them alone, whose existence is contemplated by the genius of the species for reasons to us impenetrable, since they are the very essence of the thing-in-itself.  Or more strictly speaking, the will to live desires to objectivise itself in an individual which is precisely determined, and can only be begotten by this particular father and this particular mother.  This metaphysical yearning of the will in itself has immediately, as its sphere of action in the circle of human beings, the hearts of the future parents, who accordingly are seized with this desire.  They now fancy that it is for their own sakes they are longing for what at present has purely a metaphysical end, that is to say, for what does not come within the range of things that exist in reality.  In other words, it is the desire of the future individual to enter existence, which has first become possible here, a longing which proceeds from the primary source of all being and exhibits itself in the phenomenal world as the intense love of the future parents for each other, and has little regard for anything outside itself.  In fact, love is an illusion like no other; it will induce a man to sacrifice everything he possesses in the world, in order to obtain this woman, who in reality will satisfy him no more than any other.  It also ceases to exist when the end, which was in reality metaphysical, has been frustrated perhaps by the woman’s barrenness (which, according to Hufeland, is the result of nineteen accidental defects in the constitution), just as it is frustrated daily in millions of crushed germs in which the same metaphysical life-principle struggles to exist; there is no other consolation in this than that there is an infinity of space, time, and matter, and consequently inexhaustible opportunity, at the service of the will to live.