 Microeconomics focuses on relations among individuals (with firms and households frequently counting as honorary individuals and little said about the idiosyncrasies of the demand of particular individuals). Individuals have complete and transitive preferences that govern their choices. Consumers prefer more commodities to fewer and have “diminishing marginal rates of substitution” — i. e. they will pay less for units of a commodity when they already have lots of it than when they have little of it. Firms attempt to maximize profits in the face of diminishing returns: holding fixed all the inputs into production except one, output increases when there is more of the remaining input, but at a diminishing rate. Economists idealize and suppose that in competitive markets, firms and individuals cannot influence prices, but economists are also interested in strategic interactions, in which the rational choices of separate individuals are interdependent. Game theory, which is devoted to the study of strategic interactions, is of growing importance in economics. Economists model the outcome of the profit-maximizing activities of firms and the attempts of consumers optimally to satisfy their preferences as an equilibrium in which there is no excess demand on any market. What this means is that anyone who wants to buy anything at the going market price is able to do so. There is no excess demand, and unless a good is free, there is no excess supply.