no satisfactory explanation of this perversion has been put forward and that it seems possible that a number of mental impulses are combined in it to produce a single resultant. But the most remarkable feature of this perversion is that its active and passive forms are habitually found to occur together in the same individual.
Freud establishes that sadism and masochism represent the paradigmatic perversion structure — active and passive forms are always latently co-present in the same subject, pointing to the reversibility of all component instincts.
, Three Essays on the Theory of Sexuality, 1905thesis