Correlative to this philosophy was Logic, or the science of the laws of thinking (Greek logos--reason, word). It taught how men should use their reason, how they should express themselves reasonably, how concepts arise (in what manner, for example, the human understanding arrived at the concepts stone, tree, animal, man, virtue, vice, etc.); further, how such concepts are combined into judgments (propositions), and finally, how conclusions are drawn from these judgments. This logic exhibited the intellectual processes of the human mind. It was founded by the Greek philosopher, Aristotle (384 to 322 B.C.), and remained essentially unaltered until the beginning of the nineteenth century, in the same way as our whole conception of the universe remained unchanged. This science of human intellectual processes was based on three original laws of thought, which best characterise it. Just as an examining magistrate looks a prisoner in the face, and identifies him, so that uncertainty and contradiction may be avoided, so this logic began by establishing the identity of the conceptions with which it was to operate. Consequently, it established as the first law of thought the Principle of Identity, which runs as follows: A = A, i.e., each thing, each being, is like itself; it possesses an individuality of its own, peculiar to itself. To put it more clearly, this principle affirms that the earth is the earth, a state is a state, Capital is Capital, Socialism is Socialism. From this proceeds the second law of thought, the Principle of Contradiction. A cannot be A and not--A. Or following our example given above, the earth cannot be the earth and a ball of fire; a State cannot be a State and an Anarchy; Capital cannot be Capital and Poverty; Socialism cannot be Socialism and Individualism. Therefore there must be no contradictions, for a thing which contradicts itself is nonsense; where, however, this occurs either in actuality or in thought, it is only an accidental exception to the rule, as it were, or a passing and irregular phenomenon. From this law of thought follows directly the third, viz., the Principle of the Excluded Middle. A thing is either A or non-A; there is no middle term. Or, according to our example, the earth is either a solid body, or, if it is not solid, it is no earth; there is no middle term. The State is either monarchical, or, if it is not monarchical, it is no State. Capitalism is either oppressive, or altogether not Capitalism. Socialism is either revolutionary, or not Socialism at all; there is no middle term. (Socialism is either reformist, or not Socialism at all; there is no middle term.) With these three intellectual laws of identity, of contradiction, and of the excluded middle, formal logic begins.