Even in the liberal and comprehensive history of the Greek philosophy by Zeller (vol. ii. p. 187, ed. 2nd), respecting Eukleides’ and the Megarians; — “Dagegen bot der Streit gegen die geltenden Meinungen dem Scharfsinn, der Rechthaberei, und dem wissenschaftlichen Ehrgeiz, ein unerschöpfliches Feld dar, welches denn auch die Megarischen Philosophen rüstig ausbeuteten.”
If by “die geltenden Meinungen” Zeller means the common sense of the day that is, the opinions and beliefs current among the ἰδιῶται, the working, enjoying, non-theorising public — it is very true that the Megaric philosophers contended against them: but Sokrates and Plato contended against them quite as much: we see this in the Platonic Apology, Gorgias, Republic, Timæus, Parmenidês, &c.
If, on the other hand, by “die geltenden Meinungen” Zeller means any philosophical or logical theories generally or universally admitted by thinking men as valid, the answer is that there were none such in the fourth and third centuries B.C. Various eminent speculative individuals were labouring to construct such theories, each in his own way, and each with a certain congregation of partisans; but established theory there was none. Nor can any theory (whether accepted or not) be firm or trustworthy, unless it be exposed to the continued thrusts of the negative weapon, searching out its vulnerable points. We know of the Megarics only what they furnished towards that negative testing; without which, however, — as we may learn from Plato and Aristotle themselves, — the true value of the affirmative defences can never be measured.
Whoever undertakes, bonâ fide, to frame a complete and defensible logical theory, will desire to have before him a copious collection of such difficulties, and will consider those who propound them as useful auxiliaries.[45] If he finds no one to propound them, he will have to imagine them for himself. “The philosophy of reasoning” (observes Mr. John Stuart Mill) “must comprise the philosophy of bad as well as of good reasoning.”[46] The one cannot be complete without the other. To enumerate the different varieties of apparent evidence which is not real evidence (called Fallacies), and of apparent contradictions which are not real contradictions — referred as far as may be to classes, each illustrated by a suitable type — is among the duties of a logician. He will find this duty much facilitated, if there happen to exist around him an active habit of dialectic debate: ingenious men who really study the modes of puzzling and confuting a well-armed adversary, as well as of defending themselves against the like. Such a habit did exist at Athens: and unless it had existed, the Aristotelian theories on logic would probably never have been framed. Contemporary and antecedent dialecticians, the Megarici among them, supplied the stock of particular examples enumerated and criticised by Aristotle in the Topica:[47] which treatise (especially the last book, De Sophisticis Elenchis) is intended both to explain the theory, and to give suggestions on the practice, of logical controversy. A man who takes lessons in fencing must learn not only how to thrust and parry, but also how to impose on his opponent by feints, and to meet the feints employed against himself: a general who learns the art of war must know how to take advantage of the enemy by effective cheating and treachery (to use the language of Xenophon), and how to avoid being cheated himself. The Aristotelian Topica, in like manner, teach the arts both of dialectic attack and of dialectic defence.[48]
[45] Marbach (Gesch. der Philos. s. 91), though he treats the Megarics as jesters (which I do not think they were), yet adds very justly: “Nevertheless these puzzles (propounded by the Megarics) have their serious and scientific side. We are forced to inquire, how it happens that the contradictions shown up in them are not merely possible but even necessary.”
Both Tiedemann and Winckelmann also remark that the debaters called Eristics contributed greatly to the formation of the theory and precepts of Logic, afterwards laid out by Aristotle. Winckelmann, Prolegg. ad Platon. Euthydem. pp. xxiv.-xxxi. Even Stallbaum, though full of harshness towards those Sophists whom he describes as belonging to the school of Protagoras, treats the Megaric philosophers with much greater respect. Prolegom. ad Platon. Euthydem. p. 9.
[46] System of Logic, Book v. 1, 1.
[47] Prantl (Gesch. der Logik, vol. i. pp. 43-50) ascribes to the Megarics all or nearly all the sophisms which Aristotle notices in the Treatise De Sophisticis Elenchis. This is more than can be proved, and more than I think probable. Several of them are taken from the Platonic Euthydêmus.
[48] See the remarkable passages in the discourses of Sokrates (Memorab. iii. 1, 6; iv. 2, 15), and in that of Kambyses to Cyrus, which repeats the same opinion — Cyropæd. i. 6, 27 — respecting the amount of deceit, treachery, the thievish and rapacious qualities required for conducting war against an enemy — (τὰ πρὸς τοὺς πολεμίους νόμιμα, i. 6, 34).
Aristotle treats of Dialectic, as he does of Rhetoric, as an art having its theory, and precepts founded upon that theory. I shall have occasion to observe in a future chapter ([xxi].), that logical Fallacies are not generated or invented by persons called Sophists, but are inherent liabilities to error in the human intellect; and that the habit of debate affords the only means of bringing them into clear daylight, and guarding against being deceived by them. Aristotle gives precepts both how to thrust, and how to parry with the best effect: if he had taught only how to parry, he would have left out one-half of the art.