We shall see more fully how Aristotle deals with Dialectic, when we come to the Topica: here I put it forward briefly, in order that the reader may better understand, by contrast, its extreme antithesis, viz., Demonstrative Science and Necessary Truth as conceived by Aristotle. First, instead of two debaters, one of whom sets up a thesis which he professes to understand and undertakes to maintain, while the other puts questions upon it,--Demonstrative Science assumes a teacher who knows, and a learner conscious of ignorance but wishing to know. The teacher lays down premisses which the learner is bound to receive; or if they are put in the form of questions, the learner must answer them as the teacher expects, not according to his own knowledge. Secondly, instead of the unbounded miscellany of subjects treated in Dialectic, Demonstrative Science is confined to a few special subjects, in which alone appropriate premisses can be obtained, and definitions framed. Thirdly, instead of the several heterogeneous authorities recognized in Dialectic, Demonstrative Science has _principia_ of its own, serving as points of departure; some _principia_ common to all its varieties, others special or confined to one alone. Fourthly, there is no conflict of authorities in Demonstrative Science; its propositions are essential, universal, and true _per se_, from the commencement to the conclusion; while Dialectic takes in accidental premisses as well as essential. Fifthly, the _principia_ of Demonstrative Science are obtained from Induction only; originating in particulars which are all that the ordinary growing mind can at first apprehend (_notiora nobis_), but culminating in universals which correspond to the perfection of our cognitive comprehension (_notiora naturâ_.)[6] [Footnote 6: Aristot. Topica, VI. iv. p. 141, b. 3-14. [Greek: oi( polloi\ ga\r ta\ toiau=ta prognôri/zousin; ta\ me\n ga\r tê=s tuchou/sês, ta\ d' a)kribou=s kai\ perittê=s dianoi/as katamathei=n e)sti/n.] Compare in Analyt. Post. I. xii. pp. 77-78, the contrast between [Greek: ta\ mathê/mata] and [Greek: oi( dia/logoi].]