That cruelty and sexual impulse are most intimately connected is beyond
doubt taught by the history of civilization, but in the explanation of
this connection no one has gone beyond the accentuation of the
aggressive factors of the libido. The aggression which is mixed with the
sexual impulse is according to some authors a remnant of cannibalistic
lust, a participation on the part of the domination apparatus
(Bemächtigungsapparatus), which served also for the gratification of the
great wants of the other, ontogenetically the older impulse.[21] It has
also been claimed that every pain contains in itself the possibility of
a pleasurable sensation. Let us be satisfied with the impression that
the explanation of this perversion is by no means satisfactory and that
it is possible that many psychic efforts unite themselves into one
effect.

The most striking peculiarity of this perversion lies in the fact that
its active and passive forms are regularly encountered together in the
same person. He who experiences pleasure by causing pain to others in
sexual relations is also able to experience the pain emanating from
sexual relations as pleasure. A sadist is simultaneously a masochist,
though either the active or the passive side of the perversion may be
more strongly developed and thus represent his preponderate sexual
activity.[22]

We thus see that certain perverted propensities regularly appear in
_contrasting pairs_, a thing which, in view of the material to be
produced later, must claim great theoretical value. It is furthermore
clear that the existence of the contrast, sadism and masochism, can not
readily be attributed to the mixture of aggression. On the other hand
one may be tempted to connect such simultaneously existing contrasts