in the literary history of the ancients, that branch of logic which taught the rules and modes of reasoning. See Logic.

Zeno Eleates was the first who discovered the natural Dialectics series of principles and conclusions observed in reasoning, and formed a system of these in form of a dialogue, which for this reason was called dialectica.

The dialectica of the ancients is usually divided into several kinds. The first was the elocitica, or that of Zeno Eleates, which was threefold, namely; consecutionum, colloquitionum, and contentionum. The first branch consisted of rules for deducing or drawing conclusions. The second embraced the art of dialogue, which became of such universal use in philosophy that all reasoning was called interrogation; when syllogism was laid aside, the philosophers did all by dialogue, it being incumbent on the respondent to conclude and argue from the several concessions made. The last part of Zeno's dialectics, sperre, was contentious, or the art of disputing and contradicting; though some, particularly Laertius, ascribe this part to Protagoras, a disciple of Zeno.

The second kind is the dialectica megariana, the author of which is Euclid, not the mathematician, but another of Megara. He chiefly followed the method of Zeno and Protagoras, though there are two things appropriated to him; the first, that he impugned the demonstrations of others, not by assumptions, but conclusions, proceeding from consequence to consequence; and the second, that he set aside as invalid all arguments drawn from comparisons of similitude.

He was succeeded by Eubulides, from whom the sophistic mode of reasoning is said to be derived. In his time the art is described as manifold; mentiens, fallens, eleutra, obelota, arcevalis, cornuta, and calva.

The third is the dialectics of Plato, which he proposes as a kind of analysis, in order to direct the human mind, by dividing, defining, and bringing things to the simplest form; where having arrived, and stopped a little, it applies itself to explain sensible things, but with a view to return to the first truth, where alone it can rest. Such is the idea of Plato's analysis.

The fourth is Aristotle's dialectics, containing the doctrine of simple words, delivered in his book of Predicaments; the doctrine of propositions, contained in his book De Interpretatione; and that of the several kinds of syllogism, explained in his books of Analytics, Topics, and Elenchuses.

The fifth is the dialectics of the Stoics, which they call a part of philosophy, and divide into rhetoric and dialectic, to which some add the definitive, by which things are justly defined; comprehending likewise the canons or criterions of truth.

The Stoics, before they come to treat of syllogisms, consider two principal topics; the one about the signification of words, the other about the things signified. In reference to the former, they consider abundance of things belonging to the grammarian's province; as what, and how many letters, what is a word, diction, speech, &c. And, in reference to the latter, they consider things themselves, not as without the mind, but as in it, or received in it by means of the senses. Accordingly, they first teach, that nihil sit in intellectu, quod non prius fuerit in senso, whatever is in the mind came thither by the senses; and that aut incertione sui, as Plato, who meets the sight, aut similitudine, as Caesar by his effigy, aut proportione, either by enlarging as a giant, or by diminishing as a pigmy, aut translatione, as a Cyclops, aut compositione, as a Centaur, aut contrario, as death, aut privazione, as a blind man.

The sixth is the dialectic of Epicurus, who, though he affected to despise it, yet cultivated it with vigour. He was only adverse to that of the Stoics, who he thought attributed too much to it, in pronouncing him alone wise who was well versed in dialectics. For this reason Epicurus, though he appeared to set aside the common dialectics, had recourse to another way, namely, to certain canons, which he substituted in their stead; and as all questions in philosophy are either de re or de roce, he gave separate rules for each.