Both Plato and Aristotle taught that reason alone could form science; but 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. And he maintained that the individual things alone _existed_, and if individuals only exist, they can only be known by _sensation_. Sensation thus becomes the basis of knowledge. Plato made reason the basis of knowledge, but Aristotle made _experience_. Plato directed man to the contemplation of ideas; Aristotle, to the observations 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 commenced with him, since he maintained that experience furnishes the principles of every science; but, while his conception was just, there was not sufficient experience then accumulated from which to generalize with effect. He did not sufficiently verify his premises. His reasoning was correct upon the data given, as in the famous syllogism, "All black birds are crows; this bird is black; therefore this bird is a crow." The defect of the syllogism is not in the reasoning, but in the truth of the major premise, since all black birds are not crows. It is only a most extensive and exhaustive examination of the accuracy of a proposition which will warrant reasoning upon it. Aristotle reasoned without sufficient examination of the major premise of his syllogisms. [Sidenote: Logic of Aristotle.]