At that time little thought had been bestowed upon classification as a logical process.
The general necessity of systematic classification — of generalisation and specification, or subordination of species and sub-species, as a condition of knowing any extensive group of individuals — requires no advocate at the present day. But it was otherwise in the time of Plato. There existed then no body of knowledge, distributed and classified, to which he could appeal as an example. The illustrations to which he himself refers here, of language and music as systematic arrangements of vocal sounds, were both of them the product of empirical analogy and unconscious growth, involving little of predetermined principle or theory. All the classification then employed was merely that which is included in the structure of language: in the framing of general names, each designating a multitude of individuals. All that men knew of classification was, that which is involved in calling many individuals by the same common name. This is the defect pointed out by Plato, when he remarks that the clever men of his time took no heed except of the One and the Infinite (Genus and Individuals): neglecting all the intermediate distinctions. Upon the knowledge of these media (he says) rests the difference between true dialectic debate, and mere polemic.[20] That is — when you have only an infinite multitude of individuals, called by the same generic name, it is not even certain that they have a single property in common: and even if they have, it is not safe to reason from one to another as to the possession of any other property beyond the one generic property — so that the debate ends in mere perplexity. All pleasures agree in being pleasures (Sokrates had before observed to Protarchus), and all cognitions agree in being cognitions. But you cannot from hence infer that there is any other property belonging in common to all.[21] That is a point which you cannot determine without farther observation of individuals, and discrimination of the great multitude into appropriate subdivisions. You will thus bring the whole under that triple point of view which Plato requires:— the highest Genus, — the definite number of species and sub-species, — the undefined number of individuals.
[20] Plato, Philêbus, p. 17 A. οἱ δὲ νῦν τῶν ἀνθρώπων σοφοὶ ἓν μέν, ὅπως ἂν τύχωσι, καὶ πολλὰ θᾶττον καὶ βραδύτερον ποιοῦσι τοῦ δέοντος, μετὰ δὲ τὸ ἓν ἄπειρα εὐθύς, τὰ δὲ μέσα αὐτοὺς ἐκφεύγει, οἷς διακεχώρισται τό τε διαλεκτικῶς πάλον καὶ τὸ ἐριστικῶς ἡμᾶς ποιεῖσθαι πρὸς ἀλλήλους τοὺς λόγους.
[21] Plato, Philêbus, pp. 13 B, 14 A.
Classification — unconscious and conscious.
Here we have set before us one important branch of logical method — the necessity of classification, not simply arising as an incidental and unconscious effect of the transitive employment of a common name, but undertaken consciously and intentionally as a deliberate process, and framed upon principles predetermined as essential to the accomplishment of a scientific end. This was a conception new in the Sokratic age. Plato seized upon it with ardour. He has not only emphatically insisted upon it in the Philêbus and elsewhere, but he has also given (in the Sophistês and Politikus) elaborate examples of systematic logical subdivision applied to given subjects.
Plato’s doctrine about classification is not necessarily connected with his Theory of Ideas.
We may here remark that Plato’s views as to the necessity of systematic classification, or of connecting the Summum Genus with individuals by intermediate stages of gradually decreasing generality — are not necessarily connected with his peculiar theory of Ideas as Self-existent objects, eternal and unchangeable. The two are indeed blended together in his own mind and language: but the one is quite separable from the other; and his remarks on classification are more perspicuous without his theory of Ideas than with it. Classification does not depend upon his hypothesis — That Ideas are not simply Concepts of the Reason, but absolute existences apart from the Reason (Entia Rationis apart from the Ratio) — and that these Ideas correspond to the words Unum, Multa definité, Multa indefinité, which are put together to compose the totality of what we see and feel in the Kosmos.
Applying this general doctrine (about the necessity of establishing subordinate classes as intermediate between the Genus and Individuals) to the particular subject debated between Sokrates and Protarchus — the next step in the procedure would naturally be, to distinguish the subordinate classes comprised first under the Genus Pleasure — next, under the Genus Intelligence (or Cognition). And so indeed the dialogue seems to promise[22] in tolerably explicit terms.
[22] Plato, Philêbus, p. 19 B, p. 20 A.