In the later 20th century, BDSM activists have protested against these conceptual models, as they were derived from the philosophies of two singular historical figures. Both Freud and Krafft-Ebing were psychiatrists; their observations on sadism and masochism were dependent on psychiatric patients, and their models were built on the assumption of psychopathology. BDSM activists argue that it is illogical to attribute human behavioural phenomena as complex as sadism and masochism to the "inventions" of two historic individuals. Advocates of BDSM have sought to distinguish themselves from widely held notions of antiquated psychiatric theory by the adoption of the term "BDSM" as a distinction from the now common usage of those psychological terms, abbreviated as "S&M".