91 Plato, Theætêt. p. 165 D. The reasonings here given by Plato from the mouth of Sokrates, are compared by Steinhart to the Trug-schlüsse, which in the Euthydêmus he ascribes to that Sophist and Dionysodorus. But Steinhart says that Plato is here reasoning in the style of Protagoras: an assertion thoroughly gratuitous, for which there is no evidence at all (Steinhart, Einleitung zum Theætêt. p. 53).

92 Plato, Theætêt. pp. 179-183. The description which we read here (put into the mouth of the geometer Theodôrus) of the persons in Ephesus and other parts of Ionia, who speculated in the vein of Herakleitus — is full of vivid fancy and smartness, but is for that reason the less to be trusted as accurate.

The characteristic features ascribed to these Herakleiteans are quite unlike to the features of Protagoras, so far as we know them; though Protagoras, nevertheless, throughout this dialogue, is spoken of as if he were an Herakleitean. These men are here depicted as half mad — incapable of continuous attention — hating all systematic speech and debate — answering, when addressed, only in brief, symbolical, enigmatic phrases, of which they had a quiver-full, but which they never condescended to explain (ὥσπερ ἐκ φαρέτρας ῥηματίσκια αἰνιγματώδη ἀνασπῶντες ἀποτοξεύουσιν, see Lassalle, vol. i. pp. 32-39 — springing up by spontaneous inspiration, despising instruction, p. 180 A), and each looking down upon the others as ignorant. If we compare the picture thus given by Plato of the Herakleiteans, with the picture which he gives of Protagoras in the dialogue so called, we shall see that the two are as unlike as possible.

Lassalle, in his elaborate work on the philosophy of Herakleitus, attempts to establish the philosophical affinity between Herakleitus and Protagoras: but in my judgment unsuccessfully. According to Lassalle’s own representation of the doctrine of Herakleitus, it is altogether opposed to the most eminent Protagorean doctrine, Ἄνθρωπος ἑαυτῷ μέτρον — and equally opposed to that which Plato seems to imply as Protagorean — Αἴσθησις = Ἐπιστήμη. The elucidation given by Lassalle of Herakleitus, through the analogy of Hegel, is certainly curious and instructive. The Absolute Process of Herakleitus is at variance with Protagoras, not less than the Absolute Object or Substratum of the Eleates, or the Absolute Ideas of Plato. Lassalle admits that Herakleitus is the entire antithesis to Protagoras, yet still contends that he is the prior stage of transition towards Protagoras (vol. i. p. 64).

93 Plato, Theætêt. p. 184 D. δεινὸν γάρ που, εἰ πολλαί τινες ἐν ἡμῖν, ὥσπερ ἐν δουρείοις ἵπποις, αἰσθήσις ἐγκαθηνται, ἀλλὰ μὴ εἰς μίαν τινὰ ἰδέαν, εἴτε ψυχὴν εἴτε ὅ, τι δεῖ καλεῖν, πάντα ταῦτα ξυντείνει, ᾗ διὰ τούτων οἷον ὀργάνων αἰσθανόμεθα ὅσα αἰσθητά.

94 Plato, Theætêt. p. 184-185.

95 Plato, Theætêt. p. 185 D. δοκεῖ τὴν ἀρχὴν οὐδ’ εἶναι τοιοῦτον οὐδὲν τούτοις ὄργανον ἴδιον, ὥσπερ ἐκείνοις, ἀλλ’ αὐτὴ δι’ αὑτῆς ἡ ψυχὴ τὰ κοινά μοι φαίνεται περὶ πάντων ἐπισκοπεῖν.

96 Plato, Theætêt. p. 186 B. Τὴν δέ γε οὐσίαν καὶ ὅ τι ἔστον καὶ τὴν ἐναντιότητα πρὸς ἀλλήλω (of hardness and softness) καὶ τὴν οὐσίαν αὖ τῆς ἐναντιότητος, αὐτὴ ἡ ψυχὴ ἐπανιοῦσα καὶ ξυμβάλλουσα πρὸς ἄλληλα κρίνειν πειρᾶται ἡμῖν … Οὐκοῦν τὰ μὲν εὐθὺς γενομένοις πάρεστι φύσει αἰσθάνεσθαι ἀνθρώποις τε καὶ θηρίοις, ὅσα διὰ τοῦ σώματος παθήματα ἐπὶ τὴν ψυχὴν τείνει· τὰ δὲ περὶ τούτων ἀναλογίσματα, πρός τε οὐσίαν καὶ ὠφελείαν μόγις καὶ ἐν πολλῷ χρόνῳ διὰ πολλῶν πραγμάτων καὶ παιδείας παραγίγνεται, οἷς ἂν καὶ παραγίγνηται.

97 Plato, Theætêt. p. 186 D. ἐν μὲν ἄρα τοῖς παθήμασιν οὐκ ἔνι ἐπιστήμη, ἐν δὲ τῷ περὶ ἐκείνων συλλογισμῷ· οὐσίας γὰρ καὶ ἀληθείας ἐνταῦθα μέν, ὡς ἔοικε, δυνατὸν ἅψασθαι, ἐκεῖ δὲ ἀδύνατον. The term συλλογισμὸς is here interesting, before it had received that technical sense which it has borne from Aristotle downwards. Mr. Campbell explains it properly as “nearly equivalent to abstraction and generalisation” (Preface to Theætêtus, p. lxxiv., also note, p. 144).

98 Plato, Theætêt. p. 186 A. καλὸν καὶ αἰσχρόν, καὶ ἀγαθὸν καὶ κακόν. Καὶ τούτων μοι δοκεῖ ἐν τοῖς μάλιστα πρὸς ἄλληλα σκοπεῖσθαι τὴν οὐσίαν, ἀναλογιζομένη (ἡ ψυχὴ) ἐν ἑαυτῇ τὰ γεγονότα καὶ τὰ παρόντα πρὸς τὰ μέλλοντα.

Base and honourable, evil and good, are here pointed out by Sokrates as most evidently and emphatically relative. In the train of reasoning here terminated, Plato had been combating the doctrine Αἴσθησις = Ἐπιστήμη. In his sense of the word αἴσθησις he has refuted the doctrine. But what about the other doctrine, which he declares to be a part of the same programme — Homo Mensura — the Protagorean formula? That formula, so far from being refuted, is actually sustained and established by this train of reasoning. Plato has declared οὐσία, ἀληθεία, ἐναντιότης, ἀγαθόν, κακόν, &c., to be a distinct class of Objects not perceived by Sense. But he also tells us that they are apprehended by the Mind through its own working, and that they are apprehended always in relation to each other. We thus see that they are just as much relative to the concipient mind, as the Objects of sense are to the percipient and sentient mind. The Subject is the correlative limit or measure (to use Protagorean phrases) of one as well as of the other. This confirms what I observed above, that the two doctrines, 1. Homo Mensura, 2. Αἴσθησις = Ἐπιστήμη, — are completely distinct and independent, though Plato has chosen to implicate or identify them.

99 The discussion in pp. 184-186-186 of the Theætêtus is interesting as the earliest attempt remaining to classify psychological phenomena. What Demokritus and others proposed with the same view — the analogy or discrepancy between τὸ αἰσθάνεσθαι and τὸ νοεῖν — we gather only from the brief notices of Aristotle and others. Plato considers himself to have established, that “cognition is not to be sought at all in sensible perception, but in that function, whatever it be, which is predicated of the mind when it busies itself per se (i.e. not through any special bodily organ) about existences” (p. 187 A). We may here remark, as to the dispute between Plato and Protagoras, that Plato here does not at all escape from the region of the Relative, or from the Protagorean formula, Homo Mensura. He passes from Mind Percipient to Mind Cogitant; but these new Entia cogitationis (as his language implies) are still relative, though relative to the Cogitant and not to the Percipient. He reduces Mind Sentient to the narrowest functions, including only each isolated impression of one or other among the five senses. When we see a clock on the wall and hear it strike twelve — we have a visual impression of black from the hands, of white from the face, and an audible impression from each stroke. But this is all (according to Plato) which we have from sense, or which addresses itself to the sentient mind. All beyond this (according to him) is apprehended by the cogitant mind: all discrimination, comparison, and relation — such as the succession, or one, two, three, &c., of the separate impressions, the likeness of one stroke to the preceding, the contrast or dissimilarity of the black with the white — even the simplest acts of discrimination or comparison belong (in Plato’s view) to mental powers beyond and apart from sense; much more, of course, apprehension of the common properties of all, and of those extreme abstractions to which we apply the words Ens and Non-Ens (τό τ’ ἐπὶ πᾶσι κοινὸν καὶ τὸ ἐπὶ τούτοις, ᾧ τὸ ἔστιν ἐπονομάζεις καὶ τὸ οὐκ ἔστιν, p. 185 C).

When Plato thus narrows the sense of αἴσθησις, it is easy to prove that ἐπιστήμη is not αἴσθησις; but I doubt whether those who affirmed this proposition intended what he here refutes. Neither unreflecting men, nor early theorizers, would distinguish the impressions of sense from the feeling of such impressions being successive, distinct from one another, resembling, &c. Mr. John Stuart Mill observes (Logic, Book i. chap. iii. sects. 10-13) — “The simplest of all relations are those expressed by the words antecedent and consequent, and by the word simultaneous. If we say dawn preceded sunrise, the fact in which the two things dawn and sunrise were jointly concerned, consisted only of the two things themselves. No third thing entered into the fact or phenomenon at all, unless indeed we choose to call the succession of the two objects a third thing; but their succession is not something added to the things themselves, it is something involved in them. To have two feelings at all, implies having them either successively or simultaneously. The relations of succession and simultaneity, of likeness and unlikeness, not being grounded on any fact or phenomenon distinct from the related objects themselves, do not admit of the same kind of analysis. But these relations, though not (like other relations) grounded on states of consciousness, are themselves states of consciousness. Resemblance is nothing but our feeling of resemblance: succession is nothing but our feeling of succession.”

By all ordinary (non-theorising) persons, these familiar relations, involved in the facts of sense, are conceived as an essential part of αἴσθησις: and are so conceived by those modern theorists who trace all our knowledge to sense — as well as (probably) by those ancient theorists who defined ἐπιστήμη to be αἴσθησις, and against whom Plato here reasons. These theorists would have said (as ordinary language recognises) — “We see the dissimilarity of the black hands from the white face of the clock; we hear the likeness of one stroke of the clock to another, and the succession of the strokes one, two, three, one after the other”.

The reasoning of Plato against these opponents is thus open to many of the remarks made by Sir William Hamilton, in the notes to his edition of Reid’s works, upon Reid’s objections against Locke and Berkeley: Reid restricted the word Sensation to a much narrower meaning than that given to it by Locke and Berkeley. “Berkeley’s Sensation” (observes S. W. Hamilton) “was equivalent to Reid’s Sensation plus Perception. This is manifest even by the passages adduced in the text” (note to p. 289). But Reid in his remarks omits to notice this difference in the meaning of the same word. The case is similar with Plato when he refutes those who held the doctrine Ἐπιστήμη = Αἴσθησις. The last-mentioned word, in his construction, includes only a part of the meaning which they attributed to it; but he takes no notice of this verbal difference. Sir William Hamilton remarks, respecting M. Royer Collard’s doctrine, which narrows prodigiously the province of Sense, — “Sense he so limits that, if rigorously carried out, no sensible perception, as no consciousness, could be brought to bear”. This is exactly true about Plato’s doctrine narrowing αἴσθησις. See Hamilton’s edit. of Reid, Appendix, p. 844.

Aristotle understands αἴσθησις — αἰσθητικὴ ψυχὴ or ζωή — as occupying a larger sphere than that which Plato assigns to them in the Theætêtus. Aristotle recognises the five separate αἰσθήσεις, each correlating with and perceiving its ἴδιον αἰσθητόν: he also recognises ἡ κοινὴ αἴσθησις — common sensation or perception — correlating with (or perceiving) τὰ κοινὰ αἰσθητά, which are motion, rest, magnitude, figure, number. The κοινὴ αἴσθησις is not a distinct or sixth sense, apart from the five, but a general power inhering in all of them. He farther recognises αἴσθησις as discriminating, judging, comparing, knowing: this characteristic, τὸ κριτοκὸν and γνωστικόν, is common to αἴσθησις, φαντασία, νόησις, and distinguishes them all from appetite — τὸ ὀρεκτικόν, κινητικόν, &c. See the first and second chapters of the third Book of the Treatise De Animâ, and the Commentary of Simplikius upon that Treatise, especially p. 56, b. Aristotle tells us that all animals ἔχει δύναμιν σύμφυτον κριτικήν, ἣν καλοῦσιν αἴσθησιν. Anal. Poster. ii. p. 99, b. 35. And Sir William Hamilton adopts a similar view, when he remarks, that Judgment is implied in every act of Consciousness.

Occasionally indeed Aristotle partitions the soul between νοῦς and ὄρεξις — Intelligence and Appetite — recognising Sense as belonging to the head of Intelligence — see De Motu Animalium, 6, p. 700, b. 20. ταῦτα δὲ πάντα ἀνάγεται εἰς νοῦν καὶ ὄρεξιν· καὶ γὰρ ἡ φαντασία καὶ ἡ αἴσθησις τὴν αὐτὴν τῷ νῷ χώραν ἔχουσι· κριτικὰ γὰρ πάντα. Compare also the Topica, ii. 4, p. 111, a. 18.

It will thus be seen that while Plato severs pointedly αἴσθησις from anything like discrimination, comparison, judgment, even in the most rudimentary form — Aristotle refuses to adopt this extreme abstraction as his basis for classifying the mental phenomena. He recognises a certain measure of discrimination, comparison, and judgment, as implicated in sensible perceptions. Moreover, that which he calls κοινὴ αἴσθησις is unknown to Plato, who isolates each sense, and indeed each act of each sense, as much as possible. Aristotle is opposed, as Plato is, to the doctrine Ἐπιστήμη = Αἴσθησις, but he employs a different manner of reasoning against it. See, inter alia, Anal. Poster. i. 31, p. 87, b. 28. He confines ἐπιστήμη to one branch of the νοητική.

The Peripatetic Straton, the disciple of Theophrastus, denied that there was any distinct line of demarcation between τὸ αἰσθάνεσθαι and τὸ νοεῖν: maintaining that the former was impossible without a certain measure of the latter. His observation is very worthy of note. Plutarch, De Solertiâ Animalium, iii. 6, p. 961 A. Καίτοι Στράτωνός γε τοῦ φυσικοῦ λόγος ἐστίν, ἀποδεικνύων ὡς οὐδ’ αἰσθάνεσθαι τοπαράπαν ἄνευ τοῦ νοεῖν ὑπάρχει· καὶ γὰρ γράμματα πολλάκις ἐπιπορευόμενα τῇ ὄψει, καὶ λόγοι προσπίπτοντες τῇ ἀκοιῇ διαλανθάνουσιν ἡμᾶς καὶ διαφεύγουσι πρὸς ἑτέροις τὸν νοῦν ἔχοντας· εἶτ’ αὖθις ἐπανῆλθε καὶ μεταθεῖ καὶ μεταδιώκει τῶν προïεμένων ἕκαστον ἀναλεγόμενος· ᾗ καὶ λέλεκται. Νοῦς ὁρῇ, καὶ νοῦς ἀκούει, τὰ δὲ ἄλλα κωφὰ καὶ τυφλά· ὡς τοῦ περὶ τὰ ὄμματα καὶ ὦτα πάθους, ἂν μὴ παρῇ τὸ φρονοῦν, αἴσθησιν οὐ ποιοῦντος.

Straton here notices that remarkable fact (unnoticed by Plato and even by Aristotle, so far as I know) in the process of association, that impressions of sense are sometimes unheeded when they occur, but force themselves upon the attention afterwards, and are recalled by the mind in the order in which they occurred at first.

100 Plato, Theæt. p. 187 A. Sokr. ὅμως δὲ τοσοῦτόν γε προβεβήκαμεν, ὥστε μὴ ζητεῖν αὐτὴν (ἐπιστήμην) ἐν αἰσθήσει τοπαράπαν, ἀλλ’ ἐν ἐκείνῳ τῷ ὀνόματι, ὅ, τι ποτ’ ἔχει ἡ ψυχή, ὅταν αὐτὴ καθ’ αὑτὴν πραγματεύηται περὶ τὰ ὄντα. Theæt. Ἀλλὰ μὴν τοῦτό γε καλεῖται, ὡς ἐγᾦμαι, δοξάζειν. Sokr. Ὀρθῶς γὰρ οἴει.

Plato is quite right in distinguishing between αἴσθησις and δόξα, looking at the point as a question of psychological classification. It appears to me, however, most probable that those who maintained the theory Ἐπιστήμη = Αἴσθησις, made no such distinction, but included that which he calls δόξα in αἴσθησις. Unfortunately we do not possess their own exposition; but it cannot have included much of psychological analysis.

101 Schleiermacher represents Plato as discriminating Knowledge (the region of infallibility, you either possess it or not) from Opinion (the region of fallibility, true or false, as the case may be) by a broad and impassable line —

“Auch hieraus erwächst eine sehr entscheidende, nur ebenfalls nicht ausdrücklich gezogene, Folgerung, dass die reine Erkenntniss gar nicht auf demselben Gebiet liegen könne mit dem Irrthum — und es in Beziehung auf sie kein Wahr und Falsch gebe, sondern nur ein Haben oder Nicht Haben.” (Schleiermacher, Einleit. zum Theæt. p. 176.)

Steinhart (in his Einleit. zum Theæt. p. 94) contests this opinion of Schleiermacher (though he seems to give the same opinion himself, p. 92). He thinks that Plato does not recognise so very marked a separation between Knowledge and Opinion: that he considers Knowledge as the last term of a series of mental processes, developed gradually according to constant laws, and ascending from Sensible Perception through Opinion to Knowledge: that the purpose of the Theætêtus is to illustrate this theory.

Ueberweg, on the contrary, defends the opinion of Schleiermacher and maintains that Steinhart is mistaken (Aechtheit und Zeit. Platon. Schriften, p. 279).

Passages may be produced from Plato’s writings to support both these views: that of Schleiermacher, as well as that of Steinhart. In Timæus, p. 51 E, the like infallibility is postulated for Νοῦς (which there represents ἐπιστήμη) as contrasted with δόξα. But I think that Steinhart ascribes to the Theætêtus more than can fairly be discovered in it. That dialogue is purely negative. It declares that ἐπιστήμη is not αἴσθησις. It then attempts to go a step farther towards the affirmative, by declaring also that ἐπιστήμη is a mental process of computation, respecting the impressions of αἴσθησις — that it is τὸ συλλογίζεσθαι, which is equivalent to τὸ δοξάζειν: compare Phædrus, 249 B. But this affirmative attempt breaks down: for Sokrates cannot explain what τὸ δοξάζειν is, nor how τὸ δοξάζειν ψευδῆ is possible; in fact he says (p. 200 B) that this cannot be explained until we know what ἐπιστήμη is. The entire result of the dialogue is negative, as the closing words proclaim emphatically. On this point many of the commentators agree — Ast, Socher, Stallbaum, Ueberweg, Zeller, &c.

Whether it be true, as Schleiermacher, with several others, thinks (Einl. pp. 184-185), that Plato intends to attack Aristippus in the first part of the dialogue, and Antisthenes in the latter part, we have no means of determining.

102 Compare Plato, Sophistes, pp. 232 E, 233 A.

103 See the remarks on the necessity of Verification, as a guarantee for the Deductive Process, in Mr. John Stuart Mill’s System of Logic, Book iii. ch. xi. s. 8. Newton puts aside his own computation or theory respecting gravity as the force which kept the moon in its orbit, because the facts reported by observers respecting the lunar motions were for some time not in harmony with it. Plato certainly would not have surrendered any συλλογισμὸς under the same respect to observed facts. Aristotle might probably have done so; but this is uncertain.

104 Plato, Theæt. p. 187 B. It is scarcely possible to translate δοξάζειν always by the same English word.

105 Plato, Theæt. p. 187 C.

106 Plato, Theæt. p. 188.