Gravitation: The tendency of all bodies to approach one another with a strength proportion to the quantity of matter they contain --the quantity of matter they contain being ascertained by the strength of their tendency to approach one another. This is a lovely and edifying illustration of how science, having made A the proof of B, makes B the proof of A.
Ambrose Bierce , Humanizing Psychiatry: The Biocognitive Model, Future Psychiatry Press, 2009, p. 57.