In the passion of love, for instance, a cause unknown to the sufferer, but which is doubtless the spring-flood of hereditary instincts accidentally l… - George Santayana

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In the passion of love, for instance, a cause unknown to the
sufferer, but which is doubtless the spring-flood of hereditary
instincts accidentally let loose, suddenly checks the young
man's gayety, dispels his random curiosity, arrests perhaps his very breath; and when he looks for a cause to explain his
suspended faculties, he can find it only in the presence or image of another being, of whose character, possibly, he knows nothing and whose beauty may not be remarkable; yet
that image pursues him everywhere, and he is dominated by
an unaccustomed tragic earnestness and a new capacity for
suffering and joy.
If the passion be strong there is no previous interest or duty that
will be remembered before it; if it be lasting the whole life may
be reorganized by it, it may impose new habits, other manners,
and another religion.
Yet what is the root of all this idealism? An irrational instinct,
normally intermittent, such as all dumb creatures share, which
has here managed to dominate a human soul and to enlist all the
mental powers in its more or less permanent service, upsetting
their usual equilibrium.
This madness, however, inspires method; and for the first time,
perhaps, in his life, the man has something to live for.