principle of generic consistency See UNIVERSALIZ -. ABILIT. principle of indifference, a rule for assigning a probability to an event based on ‘parity of reasons.’ According to the principle, when the ‘weight of reasons’ favoring one event is equal to the ‘weight of reasons’ favoring another, the two events should be assigned the same probability. When there are n mutually exclusive and collectively exhaustive events, and there is no reason to favor one over another, then we should be ‘indifferent’ and the n events should each be assigned probability 1/n (the events are equiprobable), according to the principle. This principle is usually associated with the names Bernoulli (Ars Conjectandi, 1713) and Laplace (Théorie analytique des probabilités, 1812), and was so called by J. M. Keynes (A Treatise on Probability, 1921). The principle gives probability both a subjective (‘degree of belief’) and a logical (‘partial logical entailment’) interpretation. One rationale for the principle says that in ignorance, when no reasons favor one event over another, we should assign equal probabilities. It has been countered that any assignment of probabilities at all is a claim to some knowledge. Also, several seemingly natural applications of the principle, involving non-linearly related variables, have led to some mathematical contradictions, known as Bertrand’s paradox, and pointed out by Keynes. See also BERTRAND’S PARADOX, EQUIPROBABLE , KEYNES , LAPLACE , PROBA — BILIT. E.Ee.