A man who applies his labour to the investing of objects with value by the creation of utility of some sort, can not expect such a value to be appreciated and paid for, unless where other men have the means of purchasing it. Now, of what do these means consist? Of other values of other products, likewise the fruits of industry, capital, and land. Which leads us to a conclusion that may at first appear paradoxical, namely, that it is production which opens a demand for products.

Book I, On Production, Chapter XV, p. 133 (See also: Say's Law) - A Treatise On Political Economy (Fourth Edition) (1832)