Sexual deviance is technically any deviation from the sexual norm. Sexual disease, a new diagnostic category in the eighteenth century, was classed as a syndrome and seems in retrospect to have been an iatrogenic one based more on philosophical and moral grounds than on any medical ones. The great clinician, Hermann Boerhaave's observations of sex as a causal factor in some forms of illness also fit into a new medical theory known as vitalism, based on the work of Georg Ernst Stahl as well as others. These medical authors were developing new medical theories, and concern over onanism was increasing. Onanism was particularly debilitating to those who had not yet attained puberty, because it tended to destroy the mental faculties by putting a great strain on the nervous system. The decline in male potency and sexual activities with age were indicative, according to Tissot's theory, of the dangers of having lost semen or vital fluids earlier in life.