Jevons William Stanley (1835–82), British economist, logician, and philosopher of science. In economics, he clarified the idea of value, arguing that it is a function of utility. Later theorists imitated his use of the calculus and other mathematical tools to reach theoretical results. His approach anticipated the idea of marginal utility, a notion basic in modern economics. Jevons regarded J. S. Mill’s logic as inadequate, preferring the new symbolic logic of Boole. One permanent contribution was his introduction of the concept of inclusive ‘or’, with ‘or’ meaning ‘either or, or both’. To aid in teaching the new logic of classes and propositions, Jevons invented his ‘logical piano.’ In opposition to the confidence in induction of Mill and Whewell, both of whom thought, for different reasons, that induction can arrive at exact and necessary truths, Jevons argued that science yields only approximations, and that any perfect fit between theory and observation must be grounds for suspicion that we are wrong, not for confidence that we are right. Jevons introduced probability theory to show how rival hypotheses are evaluated. He was a subjectivist, holding that probability is a measure of what a perfectly rational person would believe given the available evidence. See also INDUCTION , PROBABILITY , UTILITARIAN- IS. R.E.B.