i2 4 DEVELOPMENT AND PURPOSE CHAP. 



then appear as a species within a genus, or as a genus con- 

 taining species, or in both relations when the two points of 

 view were combined. This was the foundation of the 

 logic of deduction and of the Classificatory method of 

 systematising knowledge. On these methods successive 

 thinkers from Plato onwards built up theories of Reality 

 with the character of which we are not here concerned. 

 What is important for us is that in so doing they worked 

 out the fundamental categories of experience, defining and 

 distinguishing substance, attribute and relation, quantity 

 and quality, the various forms of causation, the contrast 

 of the universal and the particular, of the necessary and 

 contingent. Nor could these distinctions be carried far 

 without raising the problem of knowledge, the grounds 

 of belief and the principles of reasoning. The world of 

 reality, which is also that of the necessary, the universal 

 and so the eternal, matches the system of accurate know- 

 ledge demonstrable by deduction from first principles, 

 while the contingent, the changing, the indefinite, is the 

 sphere of unscientific opinion. The method of demon- 

 stration is elaborately set out in the Aristotelian logic, and 

 the relation of its first principles to experience is summarily 

 indicated. They are educed by intelligence operating upon 

 data of sense, but the logic of the operation remains 

 shadowy. 



The structure of thought in its main outlines was thus 

 revealed by the great philosophers. But meanwhile 

 another movement was on foot. As the problem of reality 

 developed it soon became clear that it must be broken up. 

 Mathematics and astronomy were making progress, and 

 Plato distinguishes five special sciences, while Aristotle 

 lays down a general theory of scientific specialisation and 

 indicates the relation of science to metaphysics. Every 

 science has its own particular field, and, in addition to the 

 principles common to all reasoning, has its own specific 

 principles, consisting in the primary definitions of its 

 subject matter. A special science is conceived as a system- 

 atic body of truth educed by syllogistic reasoning from 

 certain original definitions and axioms the ideal which 

 Euclid sought to realise in geometry. But, in fact, science 



