A main topic in welfare economics is the rational behaviour of a consumer when, faced with various prices and incomes, he has to make a choice. The theory of consumption establishes the framework in which the rationality of consumers is de?ned and the principle on which it is based. By 109], "the rationality of a consumer may be described by postulating that a consumer has a de?nite preference over all conceivable commodity bundles and that he chooses those commodity bundles that are optimal with respect to his preference subject to budgetary constraints." Samuelson's theory of revealed...