Both Plato and Aristotle taught that reason alone can form science; but, as we have said, Aristotle differed from his master respecting the theory of ideas. He did not deny to ideas a _subjective_ existence, but he did deny that they have an objective existence. He maintained that individual things alone _exist_; and if individuals alone exist, they can be known only by _sensation_. Sensation thus becomes the basis of knowledge. Plato made _reason_ the basis of knowledge, but Aristotle made _experience_ that basis. Plato directed man to the contemplation of Ideas; Aristotle, to the observation of Nature. Instead of proceeding synthetically and dialectically like Plato, he pursues an analytic course. His method is hence inductive,--the derivation of certain principles from a sum of given facts and phenomena. It would seem that positive science began with Aristotle, since he maintained that experience furnishes the principles of every science; but while his conception was just, there was not at that time a sufficient amount of experience from which to generalize with effect. It is only a most extensive and exhaustive examination of the accuracy of a proposition which will warrant secure reasoning upon it. Aristotle reasoned without sufficient certainty of the major premise of his syllogisms. Aristotle was the father of logic, and Hegel and Kant think there has been no improvement upon it since his day. This became to him the real organon of science. "He supposed it was not merely the instrument of thought, but the instrument of investigation." Hence it was futile for purposes of discovery, although important to aid processes of thought. Induction and syllogism are the two great features of his system of logic. The one sets out from particulars already known to arrive at a conclusion; the other sets out from some general principle to arrive at particulars. The latter more particularly characterized his logic, which he presented in sixteen forms, the whole evincing much ingenuity and skill in construction, and presenting at the same time a useful dialectical exercise. This syllogistic process of reasoning would be incontrovertible, if the _general_ were better known than the _particular_; but it is only by induction, which proceeds from the world of experience, that we reach the higher world of cognition. Thus Aristotle made speculation subordinate to logical distinctions, and his system, when carried out by the mediaeval Schoolmen, led to a spirit of useless quibbling. Instead of interrogating Nature they interrogated their own minds, and no great discoveries were made. From want of proper knowledge of the conditions of scientific inquiry, the method of Aristotle became fruitless for him; but it was the key by which future investigators were enabled to classify and utilize their vastly greater collection of facts and materials.