Aristotle here makes a point against Plato and his school, inasmuch as, starting from the assumption that of the world of sense there could be no knowledge, no apprehension fixed or certain, and setting over against this a world of general forms which were fixed and certain, they had nothing with which to fill this second supposed world except the data of sense as found in individuals. Plato's mistake was in confusing the mere 'this,' which is the conceived starting-point of any sensation, but which, like a mathematical point, has nothing which can be said about it, with individual objects as they exist and are known in all the manifold and, in fact, infinite relations of reality. The bare subject 'this' presents at the one extreme the same emptiness, the same mere possibility of knowledge, which is presented at the other by the bare predicate 'is.' But Plato, having an objection to the former, as representing to him the merely physical and therefore the passing and unreal, clothes it for the nonce in the various attributes which are ordinarily associated with it when we say, 'this man,' 'this horse,' only to strip them off successively as data of sensation, and so at last get, by an illusory process of {183} abstraction and generalisation, to the ultimate generality of being, which is the mere 'is' of bare predication converted into a supposed eternal substance. Aristotle was as convinced as Plato that there must be some fixed and immovable object or reality corresponding to true and certain knowledge, but with his scientific instincts he was not content to have it left in a condition of emptiness, attractive enough to the more emotional and imaginative Plato. And hence we have elsewhere quite as strong and definite statements as those quoted above about universals [316] (p. 180), to the effect that existence is in the fullest and most real sense to be predicated of _individual_ things, and that only in a secondary sense can existence be predicated of universals, in virtue of their being found in individual things. Moreover, among universals the _species_, he maintains, has more of existence in it than the genus, because it is nearer to the individual or primary existence. For if you predicate of an individual thing of what species it is, you supply a statement more full of information and more closely connected with the thing than if you predicate to what genus it belongs; for example, if asked, "What is this?" and you answer, "A man," you give more information than if you say, "A living creature."