A historical summary of the development of the notions of supply and demand will aid the exposition. It may be noticed, first of all, that supply and demand is really a very superficial formula even though an exceedingly useful one. By virtue of its superficial character, it antagonizes few other theories, and it has been the common property of almost all schools of value theory. Cost theories and utility theories, labor theories, or social value theories, all find use for it, in one form or another. It is really quite neutral and colorless, so far as the ultimate questions of value-causation are concerned. The more fundamental causal factors offered by one theory or another are commonly supposed to operate _through_ supply or demand, in price-determination. Adam Smith seems to see this more clearly than does Ricardo. Ricardo, indeed, sometimes thought of demand and supply as forces antithetical to the forces of labor-costs which he was considering. In ch. xxx of his _Principles of Political Economy and Taxation_ (ed. McCulloch, pp. 232ff.) he holds that his natural value ultimately rules, except (p. 234) in the case of monopolized articles. Supply and demand govern the prices of monopolized articles and of all articles in the short run. I do not find in Ricardo any clear statement to the effect that cost of production operates _through_ influence on supply. Neither Adam Smith nor Ricardo felt the need of very much precision in the definition of supply and demand. Smith does, indeed, distinguish "effectual" from "absolute" demand, in a well-known passage (ed. Cannan, I, p. 58), defining effectual demand as the demand of the effectual demanders, _i. e._, these who are willing to pay the "natural price" of the commodity. The term "supply" he does not use in this passage, but speaks of the "quantity which is actually brought to market," and gives as the law of market price that it is determined by the "proportion" between this quantity and the effectual demand. That much is wanting in this analysis will be sufficiently clear when the views of J. S. Mill and Cairnes are considered. Ricardo offers even less than Smith in the way of definition. The reader may compare the pages in _Ricardo's Works_ cited above, and the discussion of the demand for labor on p. 241 in the same volume.