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.

p. 49: as cited in: "Professor Boole's Mathematical theory" in: Henry Longueville Manse, Philosophical pamphlets, (1853), p. 6 - An Investigation of the Laws of Thought (1854)

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