_Definition_ has to do with the _formation of concepts_, or determinate and unequivocal meanings. The universality of such concepts, and their consequent relation to particular things, was, as we have seen, investigated at a very early date, and gave rise to the great realistic-nominalistic controversy.[184:5] A large part of the logical discussion in the Platonic dialogues is an outgrowth of the earlier "eristic," a form of disputation in favor with the sophists, and consisting in the adroit use of ambiguity.[184:6] It is natural that in its first conscious self-criticism thought should discover the need of definite terms. The perpetual importance of definition has been largely due to the great prestige in modern philosophy of the method of geometry, which was regarded by Descartes and Spinoza as the model for systems of necessary truth.
_Self-evidence_ is the principle according to which _conviction of truth follows directly from an understanding of meaning_. In the practice of his intellectual midwifery, Socrates presupposed that thought is capable of bringing forth its own certainties. And rationalism has at all times regarded truth as ultimately accredited by internal marks recognizable by reason. Such truth arrived at antecedent to acquaintance with instances is called _a priori_, as distinguished from _a posteriori_ knowledge, or observation after the fact. There can be no principles of self-evidence, but logicians have always been more or less concerned with the enumeration of alleged self-evident principles, notably those of _contradiction_ and _identity_. A philosophical interest in the mathematical method has led to a logical study of axioms, but with a view rather to their fruitfulness than their intrinsic truth. Indeed, the interest in self-evident truth has always been subordinate to the interest in systematic truth, and the discovery of first principles most commonly serves to determine the relative priority of definite concepts, or the correct point of departure for a series of inferences.
The greater part of the famous Aristotelian logic consists in a study of _inference_, or _the derivation of new knowledge from old knowledge_. Aristotle sought to set down and classify every method of advancing from premises. The most important form of inference which he defined was the _syllogism_, a scheme of reasoning to a conclusion by means of two premises having one term in common. From the premises "all men are mortal" and "Socrates is a man," one may conclude that "Socrates is mortal." This is an instance not only of the syllogism in general, but of its most important "mood," the subsumption of a particular case under a general rule. Since the decline of Aristotle's influence in philosophy there has been a notable decrease of interest in the different forms of inference; though its fundamental importance as the very bone and sinew of _reasoning_ or _deductive thinking_ has never been challenged. Its loss of preëminence is in part due to the growth of empiricism, stimulated by the writings of Lord Bacon in the seventeenth century, and fostered by the subsequent development of experimental science.
