That axiom of Metaphysicians which is termed the principle of contradiction and which affirms that it is impossible for anything to possess a quality, and in the same time not to possess it, is a consequence of the fundamental law of thought, whose expression is x²=x.

— George Boole, An Investigation of the Laws of Thought (1854)

Source: http://books.google.com/books?id=tBNLAAAAYAAJ&pg=PA62

Cited in Henry Longueville Mansel's 1853 pamphlet Philosophical Pamphlets, p. 6.

Topics: mathematics