Such is the doctrine which Plato here lays down, respecting the difference between sensible perception, and knowledge or cognition. From his time to the present day, the same topic has continued to be discussed, with different opinions on the part of philosophers. Plato’s views are interesting, as far as his language enables us to make them out. He does not agree with those who treat sensation or sensible perception (in his language, the two are not distinguished) as a bodily phenomenon, and intelligence as a mental phenomenon. He regards both as belonging to the mind or soul. He considers that the mind is sentient as well as intelligent: and moreover, that the sentient mind is the essential basis and preliminary — universal among men and animals, as well as coæval with birth — furnishing all the matter, upon which the intelligent mind has to work. He says nothing, in this dialogue, about the three distinct souls or minds (rational, courageous, and appetitive), in one and the same body, which form so capital a feature in his Timæus and Republic: nothing about eternal, self-existent, substantial Ideas, or about the pre-existence of the soul and its reminiscence as the process of acquiring knowledge. Nor does he countenance the doctrine of innate ideas, instinctive beliefs, immediate mental intuitions, internal senses, &c., which have been recognised by many philosophers. Plato supposes the intelligent mind to work altogether upon the facts of sense; to review and compare them with one another; and to compute facts present or past, with a view to the future. All this is quite different from the mental intuitions and instincts, assumed by various modern philosophers as common to all mankind. The operations, which Plato ascribes to the intelligent mind, are said to be out of the reach of the common man, and not to be attainable except by a few, with difficulty and labour. The distinctive feature of the sentient mind, according to him, is, that it operates through a special bodily organ of sense: whereas the intelligent mind has no such special bodily organ.
Different views given by Plato in other dialogues.
But this distinction, in the first place, is not consistent with Timæus — wherein Plato assigns to each of his three human souls a separate and special region of the bodily organism, as its physical basis. Nor, in the second place, is it consistent with that larger range of observed facts which the farther development of physiology has brought to view. To Plato and Aristotle the nerves and the nervous system were wholly unknown: but it is now ascertained that the optic, auditory, and other nerves of sense, are only branches of a complicated system of sensory and motory nerves, attached to the brain and spinal cord as a centre: each nerve of sense having its own special mode of excitability or manifestation. Now the physical agency whereby sensation is carried on, is, not the organ of sense alone, but the cerebral centre acting along with that organ: whereas in the intellectual and memorial processes, the agency of the cerebral centre and other internal parts of the nervous system are sufficient, without any excitement beginning at the peripheral extremity of the special organ of sense, or even though that organ be disabled. We know the intelligent mind only in an embodied condition: that is, as working along with and through its own physical agency. When Plato, therefore, says that the mind thinks, computes, compares, &c., by itself — this is true only as signifying that it does so without the initiatory stimulus of a special organ of sense; not as signifying that it does so without the central nervous force or currents — an agency essential alike to thought, to sensation, to emotion, and to appetite.
Plato’s discussion of this question here exhibits a remarkable advance in analytical psychology. The mind rises from Sensation, first to Opinion, then to Cognition.
Putting ourselves back to the Platonic period, we must recognise that the discussion of the theory Αἴσθησις = Ἐπιστήμη, as it is conducted by Plato, exhibits a remarkable advance in psychological analysis. In analysing the mental phenomena, Plato displayed much more subtlety and acuteness than his predecessors — as far at least as we have the means of appreciating the latter. It is convenient to distinguish intellect from sensation (or sensible perception) and emotion, though both of them are essential and co-ordinate parts of our mental system, and are so recognised by Plato. It is also true that the discrimination of our sensations from each other, comparisons of likeness or unlikeness between them, observation of co-existence or sequence, and apprehension of other relations between them, &c., are more properly classified as belonging to intellect than to sense. But the language of psychology is, and always has been, so indeterminate, that it is difficult to say how much any writer means to include under the terms Sense[99] — Sensation — Sensible Perception — αἴσθησις. The propositions in which our knowledge is embodied, affirm — not sensations detached and isolated, but — various relations of antecedence and consequence, likeness, difference, &c., between two or more sensations or facts of sense. We rise thus to a state of mind more complicated than simple sensation: including (along with sensation), association, memory, discrimination, comparison of sensations, abstraction, and generalisation. This is what Plato calls opinion[100] or belief; a mental process, which, though presupposing sensations and based upon them, he affirms to be carried on by the mind through itself, not through any special bodily organ. In this respect it agrees with what he calls knowledge or cognition. Opinion or belief is the lowest form, possessed in different grades by all men, of this exclusively mental process: knowledge or cognition is the highest form of the same, attained only by a select few. Both opinion, and cognition, consist in comparisons and computations made by the mind about the facts of sense. But cognition (in Plato’s view) has special marks:—
1. That it is infallible, while opinion is fallible. You have it[101] or you have it not — but there is no mistake possible.
2. That it apprehends what Plato calls the real essence of things, and real truth, which, on the contrary, Opinion does not apprehend.
3. That the person who possesses it can maintain his own consistency under cross-examination, and can test the consistency of others by cross-examining them (λόγον δοῦναι καὶ δέξασθαι).
[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.”