The doctrine of Protagoras is completely distinct from the other doctrines. The identification of them as one and the same is only constructive — the interpretation of Plato himself.
It is not improbable that the three doctrines, here put together by Plato and subjected to a common scrutiny, may have been sometimes held by the same philosophers. Nevertheless, the language[16] of Plato himself shows us that Protagoras never expressly affirmed knowledge to be sensible Perception: and that the substantial identity between this doctrine, and the different doctrine maintained by Protagoras, is to be regarded as a construction put upon the two by Plato. That the theories of Herakleitus and Empedokles differed materially from each other, we know certainly: the theory of each, moreover, differed from the doctrine of Protagoras — “Man is the measure of all things”. How this last doctrine was defended by its promulgator, we cannot say. But the defence of it noway required him to maintain — That knowledge is sensible perception. It might be consistently held by one who rejected that definition of knowledge.[17] And though Plato tries to refute both, yet the reasonings which he brings against one do not at all tell against the other.
[16] See Theætêt. p. 152 A. This is admitted (to be a construction put by Plato himself) by Steinhart in his note 7, p. 214, Einleitung zum Theætêtus, though he says that Plato’s construction is the right one.
[17] Dr. Routh, in a note upon his edition of the Euthydêmus of Plato (p. 286 C) observes:— “Protagoras docebat, Πάντων χρημάτων μέτρον ἄνθρωπον εἶναι, τῶν μὲν ὄντων, ὡς ἔστι· τῶν δὲ μὴ ὄντων, ὡς οὐκ ἔστι. Quâ quidem opinione qualitatum sensilium sine animi perceptione existentiam sustulisse videtur.”
The definition here given by Routh is correct as far as it goes, though too narrow. But it is sufficient to exhibit the Protagorean doctrine as quite distinct from the other doctrine, ὅτι ἐπιστήμη οὐκ ἄλλο τί ἐστιν ἢ αἴσθησις.
Explanation of the doctrine of Protagoras — Homo Mensura.
The Protagorean doctrine — Man is the measure of all things — is simply the presentation in complete view of a common fact — uncovering an aspect of it which the received phraseology hides. Truth and Falsehood have reference to some believing subject — and the words have no meaning except in that relation. Protagoras brings to view this subjective side of the same complex fact, of which Truth and Falsehood denote the objective side. He refuses to admit the object absolute — the pretended thing in itself — Truth without a believer. His doctrine maintains the indefeasible and necessary involution of the percipient mind in every perception — of the concipient mind in every conception — of the cognizant mind in every cognition. Farther, Protagoras acknowledges many distinct believing or knowing Subjects: and affirms that every object known must be relative to (or in his language, measured by) the knowing Subject: that every cognitum must have its cognoscens, and every cognoscibile its cognitionis capax: that the words have no meaning unless this be supposed: that these two names designate two opposite poles or aspects of the indivisible fact of cognition — actual or potential — not two factors, which are in themselves separate or separable, and which come together to make a compound product. A man cannot in any case get clear of or discard his own mind as a Subject. Self is necessarily omnipresent; concerned in every moment of consciousness, and equally concerned in all, though more distinctly attended to in some than in others.[18] The Subject, self, or Ego, is that which all our moments of consciousness have in common and alike: Object is that in which they do or may differ — although some object or other there always must be. The position laid down by Descartes — Cogito, ergo sum — might have been stated with equal truth — Cogito, ergo est (cogitatum aliquid): sum cogitans — est cogitatum — are two opposite aspects of the same indivisible mental fact — cogitatio. In some cases, doubtless, the objective aspect may absorb our attention, eclipsing the subjective: in other cases, the subjective attracts exclusive notice: but in all cases and in every act of consciousness, both are involved as co-existent and correlative. That alone exists, to every man, which stands, or is believed by him to be capable of standing, in some mode of his consciousness as an Object correlative with himself as a Subject. If he believes in its existence, his own believing mind is part and parcel of such fact of belief, not less than the object believed in: if he disbelieves it, his own disbelieving mind is the like. Consciousness in all varieties has for its two poles Subject and Object: there cannot be one of these poles without the opposite pole — north without south — any more than there can be concave without convex (to use a comparison familiar with Aristotle), or front without back: which are not two things originally different and coming into conjunction, but two different aspects of the same indivisible fact.
[18] In regard to the impossibility of carrying abstraction so far as to discard the thinking subject, see Hobbes, Computation or Logic, ch. vii. 1.
“In the teaching of natural philosophy I cannot begin better than from privation; that is, from feigning the world to be annihilated. But if such annihilation of all things be supposed, it may perhaps be asked what would remain for any man (whom only I except from this universal annihilation of things) to consider as the subject of philosophy, or at all to reason upon; or what to give names unto for ratiocination’s sake.
“I say, therefore, there would remain to that man ideas of the world, and of all such bodies as he had, before their annihilation, seen with his eyes, or perceived by any other sense; that is to say, the memory and imagination of magnitudes, motions, sounds, colours, &c., as also of their order and parts. All which things, though they be nothing but ideas and phantasms, happening internally to him that imagineth, yet they will appear as if they were external and not at all depending upon any power of the mind. And these are the things to which he would give names and subtract them from, and compound them with one another. For seeing that after the destruction of all other things I suppose man still remaining, and namely that he thinks, imagines, and remembers, there can be nothing for him to think of but what is past.… Now things may be considered, that is, be brought into account, either as internal accidents of our mind, in which manner we consider them when the question is about some faculty of the mind: or, as species of external things, not as really existing, but appearing only to exist, or to have a being without us. And in this manner we are now to consider them.”