I have already had occasion, when I touched upon the other _viri Socratici_, contemporaneous with or subsequent to Plato, to give some account of the Zenonian and Megaric dialecticians, and of their sophisms or logical puzzles, which attracted so much attention from speculative men, in the fourth and third centuries B.C. These Megarics, like the Sophists, generally receive very harsh epithets from the historian of philosophy. They took the negative side, impugned affirmative dogmas, insisted on doubts and difficulties, and started problems troublesome to solve. I have tried to show, that such disputants, far from deserving all the censure which has been poured upon them, presented one indispensable condition to the formation of any tolerable logical theory.[64] Their sophisms were challenges to the logician, indicating various forms of error and confusion, against which a theory of reasoning, in order to be sufficient, was required to guard. And the demonstrations given by Plato in the latter half of the Parmenides are challenges of the same kind: only more ingenious, elaborate, and effective, than any of those (so far as we know them) proposed by the Megarics--by Zeno, or Eukleides, or Diodorus Kronus. The Platonic Parmenides here shows, that in regard to a particular question, those who believe the affirmative, those who believe the negative, and those who believe neither--can all furnish good reasons for their respective conclusions. In each case he gives the proof confidently as being good: and whether unimpeachable or not, it is certainly very ingenious and subtle. Such demonstrations are in the spirit of Sextus Empiricus, who rests his theory of scepticism upon the general fact, that there are opposite and contradictory conclusions, both of them supported by evidence equally good: the affirmative no more worthy of belief than the negative.[65] Zeno (or, as Plato calls him, the Eleatic Palamêdes[66]) did not profess any systematic theory of scepticism; but he could prove by ingenious and varied dialectic, both the thesis and the antithesis on several points of philosophy, by reasons which few, if any, among his hearers could answer. In like manner the Platonic Parmenides enunciates his contradictory demonstrations as real logical problems, which must exercise the sagacity and hold back the forward impulse of an eager philosophical aspirant. Even if this dilemma respecting Unum Est and Unum non Est, be solved, Parmenides intimates that he has others in reserve: so that either no tenable positive result will ever be attained--or at least it will not be attained until after such an amount of sagacity and patient exercise as Sokrates himself declares to be hardly practicable.[67] Herein we may see the germ and premisses of that theory which was afterwards formally proclaimed by Ænesidemus and the professed Sceptics: the same holding back ([Greek: e)pochê\]), and protest against precipitation in dogmatising,[68] which these