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The principles of definition, the law of contradiction, the

fallacy of arguing in a circle, the distinction between the

essence and accidents of a thing or notion, between means and

ends, between causes and conditions; also the division of the

mind into the rational, concupiscent, and irascible elements, or

of pleasures and desires into necessary and unnecessary-these and

other great forms of thought are all of them to be found in the

Republic, and were probably first invented by Plato. The greatest

of all logical truths, and the one of which writers on philosophy

are most apt to lose sight, the difference between words and

things, has been most strenuously insisted on by him (cp. Rep.;

Polit.; Cratyl. 435, 436 ff), although he has not always avoided

the confusion of them in his own writings (e.g. Rep.).