In psychoanalysis, the chief governing principle of the ego, exercising control over behaviour to meet the conditions imposed by external reality, thereby acting as a moderating influence on the pleasure principle. The Irish dramatist George Bernard Shaw (1856–1950) expressed its essence with remarkable clarity and brevity in his play Man and Superman: A Comedy and a Philosophy (1901–3), in the discussion about the Life Force near the end of Act III, when the Devil asks Don Juan what the use is of knowing oneself, and Don Juan replies: ‘Why, to be able to choose the line of greatest advantage instead of yielding in the direction of the least resistance’. The reality principle involves the conversion of free energy into bound energy. Sigmund Freud (1856–1939) gave a full account of this aspect of his theory in 1915 in an article on ‘Instincts and their Vicissitudes’ (Standard Edition, XIV, pp. 117–40). See also reality-ego, reality testing.