This third part, dialectic, consisting in reason and dissertation, he treated thus: Though judgments arise from the sense, yet the judgment of truth is not in the senses. The mind alone is the judge of things, and only fit to be credited, because the mind alone sees that which is simple, uniform, and certain, which is named Idea. All sense he conceived to be obtuse and slow, and nowise able to perceive those things which seem subject to sense; those things being so minute that they cannot fall under sense; so movable and various, that nothing is one, constant and the same; all are in continual alteration and movement, and subjects of opinion only. Science he affirmed to be nowhere but in the reasons and thoughts of the mind, whose objects are ideas, whence he approved definitions of things, and applied these to whatsoever subject he discussed, discriminating things and naming them etymologically. In this consisted the discipline of dialectic; that is, of speech concluded by reason. Though Socrates practised conversation by way of question and answer, or dialogue, yet Plato so much refined the form, both in speech and composition, that he deserves to be preferred before others, as well for invention as reformation. The analytical method, which reduces the thing sought into its principle, is his invention.[17] [Footnote 17: "Plato," says Grote, "appreciated dialogue, not only as the road to a conclusion, but for the mental discipline and suggestive influences of the tentative and verifying process. It was his purpose to create in his hearers a disposition to prosecute philosophical researches of their own, and at the same time to strengthen their ability of doing this with effect. This remark is especially confirmed in the two dialogues, the Sophisticus and Politicus, wherein he defends himself against reproaches seemingly made at the time. To what does all this tend? Why do you stray so widely from your professed topic? Could you not have reached the point by a shorter road? He replies by distinctly proclaiming--that the process, with its improving influence on the mind, stands first in his thoughts, the direct conclusion of the inquiry only second; that the special topic which he discusses, though in itself important, is nevertheless chosen primarily with a view to its effect in communicating general method and dialectical aptitude, just as a schoolmaster, when he gives out to his pupils a word to be spelt, looks mainly, not to the exactness in spelling the particular word, but to their command of good spelling generally. To form inquisitive testing minds, fond of philosophical debate as a pursuit, and looking at opinions on the negative as on the positive side, is the first object of most of Plato's dialogues: to teach positive truth, is only a secondary object."--_Grote's Plato, Vol. II, p. 399._]