The man who professes this doctrine, however (continues Aristotle28), shows plainly by his conduct that his mind is not thus blank; that, in respect of the contradictory alternative, he does not believe either both sides or neither side, but believes one and disbelieves the other. When he feels hungry, and seeks what he knows to be palatable and wholesome, he avoids what he knows to be nasty and poisonous. He knows what is to be found in the market-place, and goes there to get it; he keeps clear of falling into a well or walking into the sea; he does not mistake a horse for a man. He may often find himself mistaken; but he shows by his conduct that he believes certain subjects to possess certain definite attributes, and not to possess others. Though we do not reach infallible truth, we obtain an approach to it, sometimes nearer, sometimes more remote; and we thus escape the extreme doctrine which forbids all definite affirmation.29

It is in this manner that Aristotle, vindicating the Maxims of Contradiction and of Excluded Middle as the highest principia of syllogistic reasoning, disposes of the two contemporaneous dogmas that were most directly incompatible with these Maxims:— (1) The dogma of Herakleitus, who denied all duration or permanence of subject, recognizing nothing but perpetual process, flux, or change, each successive moment of which involved destruction and generation implicated with each other: Is and is not are both alike and conjointly true, while neither is true separately, to the exclusion of the other;30 (2) The dogma of Anaxagoras, who did not deny fixity or permanence of subject, but held that everything was mixed up with everything; that every subject had an infinite assemblage of contrary predicates, so that neither of them could be separately affirmed or separately denied: The truth lies in a third alternative or middle, between affirmation and denial.31

Having thus refuted these dogmas to his own satisfaction, Aristotle proceeds to impugn a third doctrine which he declares to be analogous to these two and to be equally in conflict with the two syllogistic principia which he is undertaking to vindicate. This third doctrine is the “Homo Mensura” of Protagoras: Man is the measure of all things — the measure of things existent as well as of things non-existent: To each individual that is true or false which he believes to be such, and for as long as he believes it. Aristotle contends that this doctrine is homogeneous with those of Herakleitus and Anaxagoras, and must stand or fall along with them; all three being alike adverse to the Maxim of Contradiction.32 Herein he follows partially the example of Plato, who (in his TheÆtÊtus33), though not formally enunciating the Maxim of Contradiction, had declared the tenets of Protagoras to be coincident with or analogous to those of Herakleitus, and had impugned both one and the other by the same line of arguments. Protagoras agreed with Herakleitus (so Plato and Aristotle tell us) in declaring both affirmative and negative (in the contradictory alternative) to be at once and alike true; for he maintained that what any person believed was true, and that what any person disbelieved was false. Accordingly, since opinions altogether opposite and contradictory are held by different persons or by the same person at different times, both the affirmative and the negative of every Antiphasis must be held as true alike;34 in other words, all affirmations and all negations were at once true and false. Such co-existence or implication of contradictions is the main doctrine of Herakleitus.

I have already in another work,35 while analysing the Platonic dialogues TheÆtÊtus and Kratylus, criticized at some length the doctrine here laid down by Plato and Aristotle. I have endeavoured to show that the capital tenet of Protagoras is essentially distinct from the other tenets with which these two philosophers would identify it: distinct both from the dogma of Herakleitus, That everything is in unceasing flux and process, each particular moment thereof being an implication of contradictions both alike true; and distinct also from the other dogma held by others, That all cognition is sensible perception. The Protagorean tenet “Homo Mensura” is something essentially distinct from either of these two; though possibly Protagoras himself may have held the second of the two, besides his own. His tenet is nothing more than a clear and general declaration of the principle of universal Relativity. True belief and affirmation have no meaning except in relation to some believer, real or supposed; true disbelief and negation have no meaning except in relation to some disbeliever, real or supposed. When a man affirms any proposition as true, he affirms only what he (perhaps with some other persons also) believes to be true, while others may perhaps disbelieve it as falsehood. Object and Subject are inseparably implicated: we may separate them by abstraction, and reason about each apart from the other; but, as reality, they exist only locked up one with the other.

That such is and always has been the state of the fact, in regard to truth and falsehood, belief and disbelief, is matter of notoriety: Protagoras not only accepts it as a fact, but formulates it as a theory. Instead of declaring that what he (or the oracle which he consults and follows) believes to be true, is absolute truth, while that which others believe, is truth relatively to them, — he lowers his own pretensions to a level with theirs. He professes to be a measure of truth only for himself, and for such as may be satisfied with the reasons that satisfy him. Aristotle complains that this theory discourages the search for truth as hopeless, not less than the chase after flying birds.36 But, however serious such discouragement may be, we do not escape the real difficulty of the search by setting up an abstract idol and calling it Absolute Truth, without either relativity or referee; while, if we enter, as sincere and bon fide enquirers, on the search for reasoned truth or philosophy, we shall find ourselves not departing from the Protagorean canon, but involuntarily conforming to it. Aristotle, after having declared that the Maxim of Contradiction was true beyond the possibility of deception,37 but yet that there were several eminent philosophers who disallowed it, is forced to produce the best reasons in his power to remove their doubts and bring them round to his opinion. His reasons must be such as to satisfy not his own mind only, but the minds of opponents and indifferent auditors as referees. This is an appeal to other men, as judges each for himself and in his own case: it is a tacit recognition of the autonomy of each individual enquirer as a measure of truth to himself. In other words, it is a recognition of the Protagorean canon.

We know little about the opinions of Protagoras; but there was nothing in this canon necessarily at variance either with the Maxim of Contradiction or with that of Excluded Middle. Both Aristotle and Plato would have us believe that Protagoras was bound by his canon to declare every opinion to be alike false and true, because every opinion was believed by some and disbelieved by others.38 But herein they misstate his theory. He did not declare any thing to be absolutely true, or to be absolutely false. Truth and Falsehood were considered by him as always relative to some referee, and he recognized no universal or infallible referee. In his theory the necessity of some referee was distinctly enunciated, instead of being put out of sight under an ellipsis, as in the received theories and practice. And this is exactly what Plato and Aristotle omit, when they refute him. He proclaimed that each man was a measure for himself alone, and that every opinion was true to the believer, false to the disbeliever; while they criticize him as if he had said — Every opinion is alike true and false; thus leaving out the very qualification which forms the characteristic feature of his theory. They commit that fallacy which Plato shows up in the EuthydÊmus, and which Aristotle39 numbers in his list of FallaciÆ Extra Dictionem, imputing it as a vice to the Sophists: they slide À dicto secundum quid ad dictum simpliciter. And it is remarkable that Aristotle, in one portion of his argument against “Homo Mensura,” expressly admonishes the Protagoreans that they must take care to adhere constantly to this qualified mode of enunciation;40 that they must not talk of apparent truth generally, but of truth as it appears to themselves or to some other persons, now or at a different time. Protagoras hardly needed such an admonition to keep to what is the key-note and characteristic peculiarity of his own theory; since it is only by suppressing this peculiarity that his opponents make the theory seem absurd. He would by no means have disclaimed that consequence of his theory, which Aristotle urges against it as an irrefragable objection; viz., that it makes every thing relative, and recognizes nothing as absolute. This is perfectly true, and constitutes its merit in the eyes of its supporters.

Another argument of Aristotle41 against the Protagorean “Homo Mensura” — That it implies in every affirming Subject an equal authority and equal title to credence, as compared with every other affirming Subject — I have already endeavoured to combat in my review of the Platonic TheÆtÊtus, where the same argument appears fully developed. The antithesis between Plato and Aristotle on one side, and Protagoras on the other, is indeed simply that between Absolute and Relative. The Protagorean doctrine is quite distinct from the other doctrines with which they jumble it together — from those of Herakleitus and Anaxagoras, and from the theory that Knowledge is sensible perception. The real opponents of the Maxim of Contradiction were Herakleitus, Anaxagoras, Parmenides, and Plato himself as represented in some of his dialogues, especially the Parmenides, TimÆus, Republic, Sophistes. Each of these philosophers adopted a First Philosophy different from the others: but each also adopted one completely different from that of Aristotle, and not reconcileable with his logical canons. None of them admitted determinate and definable attributes belonging to determinate particular subjects, each with a certain measure of durability.

Now the common speech of mankind throughout the Hellenic world was founded on the assumption of such fixed subjects and predicates. Those who wanted information for practical guidance or security, asked for it in this form; those who desired to be understood by others, and to determine the actions of others, adopted the like mode of speech. Information was given through significant propositions, which the questioner sought to obtain, and which the answer, if cognizant, enunciated: e.g., TheÆtÊtus is sitting down42 — to repeat the minimum or skeleton of a proposition as given by Plato, requiring both subject and predicate in proper combination, to convey the meaning. Now the logical analysis, and the syllogistic precepts of Aristotle, — as well as his rhetorical and dialectical suggestions for persuading, for refuting, or for avoiding refutation — are all based upon the practice of common speech. In conversing (he says) it is impossible to produce and exhibit the actual objects signified; the speaker must be content with enunciating, instead thereof, the name significant of each.43 The first beginning of rhetorical diction is, to speak good Greek;44 the rhetor and the dialectician must dwell upon words, propositions, and opinions, not peculiar to such as have received special teaching, but common to the many and employed in familiar conversation; the auditors, to whom they address themselves, are assumed to be commonplace men, of fair average intelligence, but nothing beyond.45 Thus much of acquirement is imbibed by almost every one as he grows up, from the ordinary intercourse of society. The men of special instruction begin with it, as others do; but they also superadd other cognitions or accomplishments derived from peculiar teachers. Universally — both in the interior of the family, amidst the unscientific multitude, and by the cultivated few — habitual speech was carried on through terms assuming fixed subjects and predicates. It was this recognized process in its two varieties of Analytic and Dialectic, which Aristotle embraced in his logical theory, and to which he also adapted his First Philosophy.

But the First Philosophy that preceded his, had not been so adapted. The Greek philosophers, who flourished before dialectical discussion had become active, during the interval between Thales and Sokrates, considered Philosophy as one whole — rerum divinarum et humanarum scientia — destined to render Nature or the Kosmos more or less intelligible. They took up in the gross all those vast problems, which the religious or mythological poets had embodied in divine genealogies and had ascribed to superhuman personal agencies.

Thales and his immediate successors (like their predecessors the poets) accommodated their hypotheses to intellectual impulses and aspirations of their own; with little anxiety about giving satisfaction to others,46 still less about avoiding inconsistencies or meeting objections. Each of them fastened upon some one grand and imposing generalization (set forth often in verse) which he stretched as far as it would go by various comparisons and illustrations, but without any attention or deference to adverse facts or reasonings. Provided that his general point of view was impressive to the imagination,47 as the old religious scheme of personal agencies was to the vulgar, he did not concern himself about the conditions of proof or disproof. The data of experience were altogether falsified (as by the Pythagoreans)48 in order to accommodate them to the theory; or were set aside as deceptive and inexplicable from the theory (as by both Parmenides and Herakleitus).49

But these vague hypotheses became subjected to a new scrutiny, when the dialectical age of Zeno and Sokrates supervened. Opponents of Parmenides impugned his theory of Ens Unum Continuum Immobile, as leading to absurdities; while his disciple Zeno replied, not by any attempt to disprove such allegations but, by showing that the counter-theory of Entia Plura Discontinua Moventia, or Mutabilia, involved consequences yet more absurd.50 In the acute dialectical warfare, to which the old theories thus stood exposed, the means of attack much surpassed those of defence; moreover, the partisans of Herakleitus despised all coherent argumentation, confining themselves to obscure oracular aphorisms and multiplied metaphors.51 In point of fact, no suitable language could be found, consistently with common speech or common experience, for expanding in detail either the Herakleitean52 or the Parmenidean theory; the former suppressing all duration and recognizing nothing but events — a perpetual stream of Fientia or interchange of Ens with Non-Ens; the latter discarding Non-Ens as unmeaning, and recognizing no real events or successions, but only Ens Unum perpetually lasting and unchangeable. The other physical hypotheses, broached by Pythagoras, Empedokles, Anaxagoras, and Demokritus, each altogether discordant with the others, were alike imposing in their general enunciation and promise, alike insufficient when applied to common experience and detail.

But the great development of Dialectic during the Sokratic age, together with the new applications made of it by Sokrates and the unrivalled acuteness with which he wielded it, altered materially the position of these physical theories. Sokrates was not ignorant of them;53 but he discouraged such studies, and turned attention to other topics. He passed his whole life in public and in indiscriminate conversation with every one. He deprecated astronomy and physics as unbecoming attempts to pry into the secrets of the gods; who administered the general affairs of the Kosmos according to their own pleasure, and granted only, through the medium of prophecy or oracles, such special revelations as they thought fit. In his own discussions Sokrates dwelt only on matters of familiar conversation and experience — social, ethical, political, &c., such as were in every one’s mouth, among the daily groups of the market-place. These he declared to be the truly human topics54 — the proper study of mankind — upon which it was disgraceful to be ignorant, or to form untrue and inconsistent judgments. He found, moreover, that upon these topics no one supposed himself to be ignorant, or to require teaching. Every one gave confident opinions, derived from intercourse with society, embodied in the familiar words of the language, and imbibed almost unconsciously along with the meaning of these words. Now Sokrates not only disclaimed all purpose of teaching, but made ostentatious profession of his own ignorance. His practice was to ask information from others who professed to know; and with this view, to question them about the import of vulgar words with the social convictions contained in them.55 To the answers given he applied an acute cross-examination, which seldom failed to detect so much inconsistency and contradiction as to cover the respondent with shame, and to make him sensible that he was profoundly ignorant of matters which he had believed himself to know well. Sokrates declared, in his last speech before condemnation by the Athenian Dikasts, that such false persuasion of knowledge, combined with real ignorance, was universal among mankind; and that the exposure thereof, as the great misguiding force of human life, had been enjoined upon him as his mission by the Delphian God.56

The peculiarities which Aristotle ascribes to Sokrates are — that he talked upon ethical topics instead of physical, that he fastened especially on the definitions of general terms, and that his discussions were inductive, bringing forward many analogous illustrative or probative particulars to justify a true general proposition, and one or a few to set aside a false one.57 This Sokratic practice is copiously illustrated both by Plato in many of his dialogues, and by Xenophon throughout all the Memorabilia.58 In Plato, however, Sokrates is often introduced as spokesman of doctrines not his own; while in Xenophon we have before us the real man as he talked in the market-place, and apparently little besides. Xenophon very emphatically exhibits to us a point which in Plato’s Dialogues of Search is less conspicuously marked, though still apparent: viz., the power possessed by Sokrates of accommodating himself to the ordinary mind in all its varieties — his habit of dwelling on the homely and familiar topics of the citizen’s daily life — his constant appeal to small and even vulgar details, as the way of testing large and imposing generalities.59 Sokrates possessed to a surprising degree the art of selecting arguments really persuasive to ordinary non-theorizing men; so as often to carry their assent along with him, and still oftener to shake their previous beliefs, if unwarranted, or even if adopted by mere passive receptivity without preliminary reflection and comparison.

57 Aristot. Metaph. M. iv. p. 1078, b. 28: d?? ??? ?st?? ? t?? ?? ?p?d??? S????te? d??a???, t??? t’ ?pa?t????? ?????? ?a? t? ????es?a? ?a?????· ta?ta ??? ?st?? ?f? pe?? ????? ?p?st???. — ib. A. p. 987, b. 1: S????t??? d? pe?? ?? t? ????? p?a?ate??????, pe?? d? t?? ???? f?se?? ?????, ?? ??t?? t??t??? t? ?a????? ??t???t?? ?a? pe?? ???s?? ?p?st?sa?t?? p??t?? t?? d?????a?.