These thirteenth century thinkers maintained that the universal was real, or objective in character, one in many, as Aristotle had said, and that the particular, which was likewise real, had to be explained by reference to the universal and not the universal by reference to it. Largely on account of the difficulties it appeared to throw in the way of any doctrine of human freedom, a reaction against the realistic theory made itself felt, however, and, in the later period of Scholasticism, William of Ockham, at the beginning of the fourteenth century, insisted that, if the universal were not a substance, it could not be anything objec tively and essentially real, but that it was an "intentio mentis," a conceptual object, a thought in mente.
Thus, there was developed, in opposition to realism, the doc trine known as conceptualism, according to which the only ex istent realities are individual entities, while universals are mental constructs, concepts, "ideas" in our sense of the term, obtained by a process of abstraction from resembling individual objects. An extreme form of this doctrine found expression in the con tention that, since thought or generalisation is impossible without the use of words, the universal is a mere name (a nornen or flatus vocis), a device of language for the purpose of human inter course. Hence the designation nominalism came into vogue. It was applied originally to the extreme view just indicated, which, although not rightly ascribable to William of Ockham, was held by some Scholastics (as, for example, by Roscellin as early as the eleventh century) and in the seventeenth century by the philosopher Thomas Hobbes (1588-1679). It has since, however, become more or less customary to use the term nominalism in a wider significance, and to understand by it the general doctrine that the ultimate constituents of the world of existence are indi vidual entities and not universals. The Scholastics summed up
the principle both of conceptualism and of extreme nominalism in the formula universalia post rem.
a. The Rationalistic Theories.—Modern philosophy, par ticularly as regards the treatment of knowledge, begins with Des cartes (1596-165o). He, for the first time, definitely placed the problem of knowledge in the forefront of philosophical inquiry. The nature of the Cartesian doubt is only to be understood in the light of the contrast between the ideal of knowledge, as Des cartes conceived it, and the accepted scientific doctrines of his time. His ideal of knowledge in so far resembled Plato's that it may be termed a generalisation of the mathematical ideal. In the three features exemplified in mathematical procedure (a) simplicity and certainty of data, (b) strict proof in the connec tion between data and conclusion, and (c) completeness of sur vey, he discerned the marks of systematic knowledge and his contention was that so far from being restricted to the sphere of mathematics these features should characterise knowledge as a whole.
The main proce,sses of knowledge should be accordingly three. We should start with the intuition of simple data and axioms,— data which we can clearly and distinctly apprehend and there fore be sure of directly, propositions between the terms of which we can see immediately a necessary connection. We should pro ceed by deduction from these simple truths to the more complex notions that depend upon them, passing progressively from con dition to conditioned, and at each step being certain that the con nection is as clear and distinct as are the simple truths them Selves. And, finally, by induction or enumeration we should pass in review all the elementary factors of any complex so as to make sure that the work of deduction has been completely accomp lished. Viewed more generally, the two last processes might be named synthesis and analysis respectively.