Arguments of Sokrates in examining this question. Divergence between one man and another arises, not merely from different sensual impressibility, but from mental and associative difference.
A loose adumbration of this doctrine is here given by Plato as the doctrine of Protagoras, in the words — Knowledge is sensible perception. To sift this doctrine is announced as his main purpose;[85] and we shall see how he performs the task. Sokr. — Shall we admit, that when we perceive things by sight or hearing, we at the same time know them all? When foreigners talk to us in a strange language, are we to say that we do not hear what they say, or that we both hear and know it? When unlettered men look at an inscription, shall we contend that they do not see the writing, or that they both see and know it? Theætêt. — We shall say, under these supposed circumstances, that what we see and hear, we also know. We hear and we know the pitch and intonation of the foreigner’s voice. The unlettered man sees, and also knows, the colour, size, forms, of the letters. But that which the schoolmaster and the interpreter could tell us respecting their meaning, that we neither see, nor hear, nor know. Sokr. — Excellent, Theætêtus. I have nothing to say against your answer.[86]
[85] Plato, Theætêt. p. 163 A. εἰς γὰρ τοῦτό που πᾶς ὁ λόγος ἡμῖν ἔτεινε, καὶ τούτου χάριν τὰ πολλὰ καὶ ἄτοπα ταῦτα ἐκινήσαμεν.
[86] Plato, Theætêt. p. 163 C.
This is an important question and answer, which Plato unfortunately does not follow up. It brings to view, though without fully unfolding, the distinction between what is really perceived by sense, and what is inferred from such perception: either through resemblance or through conjunctions of past experience treasured up in memory — or both together. Without having regard to such distinction, no one can discuss satisfactorily the question under debate.[87] Plato here abandons, moreover, the subjective variety of impression which he had before noticed as the characteristic of sense:— (the wind which blows cold, and the wine which tastes sweet, to one man, but not to another). Here it is assumed that all men hear the sounds, and see the written letters alike: the divergence between one man and another arises from the different prior condition of percipient minds, differing from each other in associative and reminiscent power.
[87] I borrow here a striking passage from Dugald Stewart, which illustrates both the passage in Plato’s text, and the general question as to the relativity of Cognition. Here, the fact of relative Cognition is brought out most conspicuously on its intellectual side, not on its perceptive side. The fact of sense is the same to all, and therefore, though really relative, has more the look of an absolute; but the mental associations with that fact are different with different persons, and therefore are more obviously and palpably relative. — Dugald Stewart, First Preliminary Dissertation to Encyclopæd. Britannica, pp. 66, 8th ed.
“To this reference of the sensation of colour to the external object, I can think of nothing so analogous as the feelings we experience in surveying a library of books. We speak of the volumes piled up on its shelves as treasures or magazines of the knowledge of past ages; and contemplate them with gratitude and reverence as inexhaustible sources of instruction and delight to the mind. Even in looking at a page of print or manuscript, we are apt to say that the ideas we acquire are received by the sense of sight; and we are scarcely conscious of a metaphor when we apply this language. On such occasions we seldom recollect that nothing is perceived by the eye but a multitude of black strokes drawn upon white paper, and that it is our own acquired habits which communicate to these strokes the whole of that significancy whereby they are distinguished from the unmeaning scrawling of an infant. The knowledge which we conceive to be preserved in books, like the fragrance of a rose, or the gilding of the clouds, depends, for its existence, on the relation between the object and the percipient mind: and the only difference between the two cases is, that, in the one, this relation is the local and temporary effect of conventional habits: in the other, it is the universal and the unchangeable work of nature.… What has now been remarked with respect to written characters, may be extended very nearly to oral language. When we listen to the discourse of a public speaker, eloquence and persuasion seem to issue from his lips; and we are little aware that we ourselves infuse the soul into every word that he utters. The case is exactly the same when we enjoy the conversation of a friend. We ascribe the charm entirely to his voice and accents; but without our co-operation, its potency would vanish. How very small the comparative proportion is, which in such cases the words spoken contribute to the intellectual and moral effect, I have elsewhere endeavoured to show.”
Argument — That sensible Perception does not include memory — Probability that those who held the doctrine meant to include memory.
Sokrates turns to another argument. If knowledge be the same thing as sensible perception, then it follows, that so soon as a man ceases to see and hear, he also ceases to know. The memory of what he has seen or heard, upon that supposition, is not knowledge. But Theætêtus admits that a man who remembers what he has seen or heard does know it. Accordingly, the answer that knowledge is sensible perception, cannot be maintained.[88]
[88] Plato, Theætêt. pp. 163, 164.