=66.= In his investigation Aristotle did not altogether break with Plato’s theory of ideas, but brought them from a transcendental world into touch with common life. He held fast to the method of induction (ἐπαγωγή) from the particular to the general, and agreed that we reach the true nature of each thing when we have determined the class-conception. But the class-conception or idea (ἰδέα), though the most real existence, does not exist independently, but only in and through the particulars, which compose the class. Having thus come to see that there are gradations of existence, we need to inquire what these are; and to classify the various kinds of judgment with regard to which we inquire whether they are true or false. Now by observation we find that judgments or predications have ten different shapes, to which therefore there must correspond ten kinds of existence. These are the well-known ‘categories’ of Aristotle, and are as follows: (i) ‘substance,’ as when we say ‘this is a man,’ ‘a horse’; (ii) ‘quantity,’ as that he is ‘six feet high’; (iii) ‘quality,’ as ‘a grammarian’; (iv) ‘relation,’ as ‘twice as much’; (v) ‘place,’ as ‘at Athens’; (vi) ‘time,’ as ‘last year’; (vii) ‘position,’ as ‘lying down’; (viii) ‘possession,’ as ‘with a sword’; (ix) ‘action,’ as ‘cuts’; and (x) ‘passion,’ as ‘is cut’ or ‘is burned.’ Aristotle thus reinstates the credit of the common man; he it is who possesses the substance of truth and gives it habitual expression by speech, even roughly indicating the various kinds of existence by different forms of words. It is now indicated that a study of grammar is required as the foundation of logic. Aristotle also greatly advanced the study of that kind of reasoning which proceeds from the general to the particular, and which is best expressed in terms of the ‘syllogism’ (συλλογισμός), of which he defined the various forms. [Sidenote: The four causes.] =67.= In the study of physics Aristotle picks up the thread which Socrates had dropped deliberately, that is, the teaching of the Ionic philosophers. Either directly from Empedocles, or from a _consensus_ of opinion now fairly established, he accepted the doctrine of the four elements (στοιχεῖα), earth, water, air, and fire; but to these he added a fifth (πεμπτὸν στοιχεῖον, _quinta essentia_), the aether, which fills the celestial spaces. Behind this analysis lies the more important problem of cosmology, the question how this world comes to be. Collecting once more the opinions commonly held, Aristotle concludes that four questions are usually asked, and that in them the search is being made for four ‘causes,’ which will solve the respective questions. The four causes are: