  The fundamental mental powers for knowledge acquisition are the power to discriminate, to detect identity and to retain. The fundamental laws of thought are threefold: the Law of Identity, the Law of Contradiction and the Law of Duality. The first law is described as “Whatever is, is” and implies that a thing is always identical with itself. Jevons does not define the concept of “identity”, and tends to regard it as self-evident. The second law is the classical law of contradiction: “A thing cannot both be and not be”.  The third law is the law of the excluded middle: “A thing must either be or not be.”  Jevons suggests that these three laws are merely different aspects of one and the same Law, but considers it impossible to express this Law in less than three lines. His symbolic representation of these laws is as follows: