The real Sokrates (i.e. as he is described by Xenophon) inculcated in his conversation steady reverence for the invisible, as apart from and overriding the phenomena of sensible experience; but he interpreted the term in a religious sense, as signifying the agency of the personal gods, employed to produce effects beneficial or injurious to mankind.[5] He also puts forth his dialectical acuteness to prepare consistent and tenable definitions of familiar general terms (of which instances have already been given), at least so far as to make others feel, for the first time, that they did not understand these terms, though they had been always talking like persons that did understand. But the Platonic Sokrates (i.e. as spokesman in the dialogues of Plato) enlarges both these discussions materially. Plato recognizes, not simply the invisible persons or gods, but also a separate world of invisible, impersonal entities or objects; one of which he postulates as the objective reality, though only a cogitable reality, correlating with each general term. These Entia he considers to be not merely distinct realities, but the only true and knowable realities: they are eternal and unchangeable, manifested by the fact that particulars partake in them, and imparting a partial show of stability to the indeterminate flux of particulars: unless such separate Universal Entia be supposed, there is nothing whereon cognition can fasten, and consequently there can be no cognition at all.[6] These are the substantive, self-existent Ideas, or Forms that Plato first presented to the philosophical world; sometimes with logical acuteness, oftener still with rich poetical and imaginative colouring. They constitute the main body and characteristic of the hypothesis of Realism.
[5] Xenophon, Memorab. I. iv. 9-17; IV. iii. 14.
[6] Aristot. Metaphys. A. vi. p. 987, b. 5; M. iv. p. 1078, b. 15.
But, though the main hypothesis is the same, the accessories and manner of presentation differ materially among its different advocates. In these respects, indeed, Plato differs not only from others, but also from himself. Systematic teaching or exposition is not his purpose, nor does he ever give opinions in his own name. We have from him an aggregate of detached dialogues, in many of which this same hypothesis is brought under discussion, but in each dialogue, the spokesmen approach it from a different side; while in others (distinguished by various critics as the Sokratic dialogues) it does not come under discussion at all, Plato being content to remain upon the Sokratic platform, and to debate the meaning of general terms without postulating in correlation with them an objective reality, apart from their respective particulars.
At the close of the Platonic dialogue called Kratylus, Sokrates is introduced as presenting the hypothesis of self-existent, eternal, unchangeable Ideas (exactly in the way that Aristotle ascribes to Plato) as the counter-proposition to the theory of universal flux and change announced by Herakleitus. Particulars are ever changing (it is here argued) and are thus out of the reach of cognition; but, unless the Universal Ideas above them, such as the Self-beautiful, the Self-good, &c., be admitted as unchangeable, objective realities, there can be nothing either nameable or knowable: cognition becomes impossible.
In the Timæus, Plato describes the construction of the Kosmos by a Divine Architect, and the model followed by the latter in his work. The distinction is here again brought out, and announced as capital, between the permanent, unalterable Entia, and the transient, ever-fluctuating Fientia, which come and go, but never really are. Entia are apprehended by the cogitant or intelligent soul of the Kosmos, Fientia by the sentient or percipient soul; the cosmical soul as a whole, in order to suffice for both these tasks, is made up of diverse component elements — Idem, correlating with the first of the two, Diversum, correlating with the second, and Idem implicated with Diversum, corresponding to both in conjunction. The Divine Architect is described as constructing a Kosmos, composed both of soul and body, upon the pattern of the grand pre-existent Idea — αὐτοζῷον or the Self-Animal; which included in itself as a genus the four distinct species — celestial (gods, visible and invisible), terrestrial, aerial, and aquatic.
The main point that Plato here insists upon is — the eternal and unchangeable reality of the cogitable objects called Ideas, prior both in time and in logical order to the transient objects of sight and touch, and serving as an exemplar to which these latter are made to approximate imperfectly. He assumes such priority, without proof, in the case of the Idea of Animal; but, when he touches upon the four elements — Fire, Air, Water, Earth — he hesitates to make the same assumption, and thinks himself required to give a reason for it. The reason that he assigns (announced distinctly as his own) is as follows: If Intellection (Cogitation, Νοῦς) and true Opinion are two genera distinct from each other, there must clearly exist Forms or Ideas imperceptible to our senses, and apprehended only by cogitation or intellection; but if, as some persons think, true opinion is noway different from intellection, then we must admit all the objects perceived by our senses as firm realities. Now the fact is (he proceeds to say) that true opinion is not identical with intellection, but quite distinct, separate, and unlike to it. Intellection is communicated by teaching, through true reasoning, and is unshakeable by persuasion; true opinion is communicated by persuasion and removed by counter-persuasion, without true reasoning. True opinion may belong to any man; but intellection is the privilege only of gods and of a small section of mankind. Accordingly, since the two are distinct, the objects correlating with each of them must also be distinct from each other. There must exist, first, primary, eternal, unchangeable Forms, apprehended by intellect or cogitation, but imperceptible by sense; and, secondly, resemblances of these bearing the same name, generated and destroyed each in some place, and apprehended first by sense, afterwards by opinion. Thirdly, there must be the place wherein such resemblances are generated; a place itself imperceptible by sense, yet postulated, as a receptacle indispensable for them, by a dreamy kind of computation.
We see here that the proof given by Plato, in support of the existence of Forms as the primary realities, is essentially psychological: resting upon the fact that there is a distinct mental energy or faculty called Intellection (apart from Sense and Opinion), which must have its distinct objective correlate; and upon the farther fact, that intellection is the high prerogative of the gods, shared only by a few chosen men. This last point of the case is more largely and emphatically brought out in the Phædrus, where Sokrates delivers a highly poetical effusion respecting the partial intercommunion of the human soul with these eternal intellectual realities. To contemplate them is the constant privilege of the gods; to do so is also the aspiration of the immortal soul of man generally, in the pre-existent state, prior to incorporation with the human body; though only in a few cases is such aspiration realized. Even those few human souls, that have succeeded in getting sight of the intellectual Ideas (essences without colour, figure, or tactile properties), lose all recollection of them when first entering into partnership with a human body; but are enabled gradually to recall them, by combining repeated impressions and experience of their resemblances in the world of sense. The revival of these divine elements is an inspiration of the nature of madness; though it is a variety of madness as much better than uninspired human reason as other varieties are worse. The soul, becoming insensible to ordinary pursuits, contracts a passionate devotion to these Universal Ideas, and to that dialectical communion, especially with some pregnant youthful mind, that brings them into clear separate contemplation disengaged from the limits and confusion of sense.
Here philosophy is presented as the special inspiration of a few, whose souls during the period of pre-existence have sufficiently caught sight of the Universal Ideas or Essences; so that these last, though overlaid and buried when the soul is first plunged in a body, are yet revivable afterwards under favourable circumstances, through their imperfect copies in the world of sense; especially by the sight of personal beauty in an ingenuous and aspiring youth, in which case the visible copy makes nearest approach to the perfection of the Universal Idea or Type. At the same time, Plato again presents to us the Cogitable Universals as the only objects of true cognition, the Sensible Particulars being objects merely of opinion.
In the Phædon, Sokrates advances the same doctrine, that the perceptions of sense are full of error and confusion, and can at best suggest nothing higher than opinion; that true cogitation can never be attained except when the cogitant mind disengages itself from the body and comes into direct contemplation of the Universal Entia, objects eternal and always the same — The Self-beautiful, Self-good, Self-just, Self-great, Healthy, Strong, &c., all which objects are invisible, and can be apprehended only by the cogitation or intellect. It is this Cogitable Universal that is alone real; Sensible Particulars are not real, nor lasting, nor trustworthy. None but a few philosophers, however, can attain to such pure mental energy during this life; nor even they fully and perfectly. But they will attain it fully after death (their souls being immortal), if their lives have been passed in sober philosophical training. And their souls enjoyed it before birth during the period of pre-existence; having acquired, before junction with the body, the knowledge of these Universals, which are forgotten during childhood, but recalled in the way of Reminiscence, by sensible perceptions that make a distant approach to them. Thus, according to the Phædon and some other dialogues, all learning is merely reminiscence; the mind is brought back, by the laws of association, to the knowledge of Universal Realities that it had possessed in its state of pre-existence. Particulars of sense participate in these Universals to a certain extent, or resemble them imperfectly; and they are therefore called by the same name.