You will perceive that economy, scientifically speaking, is a very contracted science; it is in fact a sort of vague mathematics which calculates the causes and effects of man's industry, and shows how it may be best applied. There are a multitude of allied branches of knowledge connected with mans condition; the relation of these to political economy is analogous to the connexion of mechanics, astronomy, optics, sound, heat, and every other branch more or less of physical science, with pure mathematics.

Letter to Henrietta Jevons (28 February 1858), published in Letters and Journal of W. Stanley Jevons (1886), edited by Harriet A. Jevons, his wife, p. 101.