While Aristotle mentions these various dissentients, and especially Herakleitus, he seems to imagine that they were not really in earnest20 in their dissent. Yet he nevertheless goes at length into the case against them, as well as against others, who agreed with him in affirming the Maxim, but who undertook also to demonstrate it. Any such demonstration Aristotle declares to be impossible. The Maxim is assumed in all demonstrations; unless you grant it, no demonstration is valid; but it cannot be itself demonstrated. He had already laid down in the Analytica that the premisses for demonstration could not be carried back indefinitely, and that the attempt so to carry them back was unphilosophical.21 There must be some primary, undemonstrable truths; and the Maxim of Contradiction he ranks among the first. Still, though in attempting any formal demonstration of the Maxim you cannot avoid assuming the Maxim itself and thus falling into Petitio Principii, Aristotle contends that you can demonstrate it in the way of refutation,22 relatively to a given opponent, provided such opponent will not content himself with simply denying it, but will besides advance some affirmative thesis of his own, as a truth in which he believes; or provided he will even grant the fixed meaning of words, defining them in a manner significant alike to himself and to others, — each word to have either one fixed meaning, or a limited number of different meanings, clear and well defined.23 It is impossible for two persons to converse, unless each understands the other. A word which conveys to the mind not one meaning, but a multitude of unconnected meanings, is for all useful purposes unmeaning.24 If, therefore, the opponent once binds himself to an affirmative definition of any word, this definition may be truly predicated of the definitum as subject; while he must be considered as interdicting himself from predicating of the same subject the negative of that definition. But when you ask for the definition, your opponent must answer the question directly and bonâ fide. He must not enlarge his definition so as to include both the affirmative and negative of the same proposition; nor must he tack on to the real essence (declared in the definition) a multitude of unessential attributes. If he answers in this confused and perplexing manner, he must be treated as not answering at all, and as rendering philosophical discussion impossible.25 Such a mode of speaking goes to disallow any ultimate essence or determinate subject, and shuts out all predication; for there cannot be an infinite regress of predicates upon predicates, and accidents upon accidents, without arriving at an ultimate substratum — Subject or Essence.26 If, wherever you can truly affirm a predicate of any subject, you can also truly deny the same predicate of the same subject, it is manifest that all subjects are one: there is nothing to discriminate man, horse, ship, wall, &c., from each other; every one speaks truth, and every one at the same time speaks falsehood; a man believes and disbelieves the same thing at the same time; or he neither believes nor disbelieves, and then his mind is blank, like a vegetable.27

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 bonâ 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