James Mill

Demand and supply are terms related in a peculiar manner. A commodity which is supplied, is always, at the same time, a commodity which is the instrument of demand. A commodity which is the instrument of demand, is always, at the same time, a commodity added to the stock of supply. Every commodity is always, at one and the same time, matter of demand, and matter of supply. Of two men who perform an exchange, the one does not come with only a supply, the other with only a demand; each of them comes with both a demand and a supply. The supply, which he brings, is the instrument of his demand; and his demand and supply are of course exactly equal to one another.

James Mill, Elements of Political Economy (1821; 1844), Chapter 4, “Consumption” Section III, “That Consumption Is Co-Extensive With Production.”