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On the whole, we must allow that the great service rendered by Plato to metaphysics in the Sophist, is not his explanation of 'Not-being' as difference. With this he certainly laid the ghost of 'Not-being'; and we may attribute to him in a measure the credit of anticipating Spinoza and Hegel. But his conception is not clear or consistent; he does not recognize the different senses of the negative, and he confuses the different classes of Not-being with the abstract notion. As the Pre-Socratic philosopher failed to distinguish between the universal and the true, while he placed the particulars of sense under the false and apparent, so Plato appears to identify negation with falsehood, or is unable to distinguish them. The greatest service rendered by him to mental science is the recognition of the communion of classes, which, although based by him on his account of 'Not-being,' is independent of it. He clearly saw that the isolation of ideas or classes is the annihilation of reasoning. Thus, after wandering in many diverging paths, we return to common sense. And for this reason we may be inclined to do less than justice to Plato,—because the truth which he attains by a real effort of thought is to us a familiar and unconscious truism, which no one would any longer think either of doubting or examining.

IV. The later dialogues of Plato contain many references to contemporary philosophy. Both in the Theaetetus and in the Sophist he recognizes that he is in the midst of a fray; a huge irregular battle everywhere surrounds him (Theaet.). First, there are the two great philosophies going back into cosmogony and poetry: the philosophy of Heracleitus, supposed to have a poetical origin in Homer, and that of the Eleatics, which in a similar spirit he conceives to be even older than Xenophanes (compare Protag.). Still older were theories of two and three principles, hot and cold, moist and dry, which were ever marrying and being given in marriage: in speaking of these, he is probably referring to Pherecydes and the early Ionians. In the philosophy of motion there were different accounts of the relation of plurality and unity, which were supposed to be joined and severed by love and hate, some maintaining that this process was perpetually going on (e.g. Heracleitus); others (e.g. Empedocles) that there was an alternation of them. Of the Pythagoreans or of Anaxagoras he makes no distinct mention. His chief opponents are, first, Eristics or Megarians; secondly, the Materialists.

The picture which he gives of both these latter schools is indistinct; and he appears reluctant to mention the names of their teachers. Nor can we easily determine how much is to be assigned to the Cynics, how much to the Megarians, or whether the 'repellent Materialists' (Theaet.) are Cynics or Atomists, or represent some unknown phase of opinion at Athens. To the Cynics and Antisthenes is commonly attributed, on the authority of Aristotle, the denial of predication, while the Megarians are said to have been Nominalists, asserting the One Good under many names to be the true Being of Zeno and the Eleatics, and, like Zeno, employing their negative dialectic in the refutation of opponents. But the later Megarians also denied predication; and this tenet, which is attributed to all of them by Simplicius, is certainly in accordance with their over-refining philosophy. The 'tyros young and old,' of whom Plato speaks, probably include both. At any rate, we shall be safer in accepting the general description of them which he has given, and in not attempting to draw a precise line between them.

Of these Eristics, whether Cynics or Megarians, several characteristics are found in Plato:—

1. They pursue verbal oppositions; 2. they make reasoning impossible by their over-accuracy in the use of language; 3. they deny predication; 4. they go from unity to plurality, without passing through the intermediate stages; 5. they refuse to attribute motion or power to Being; 6. they are the enemies of sense;—whether they are the 'friends of ideas,' who carry on the polemic against sense, is uncertain; probably under this remarkable expression Plato designates those who more nearly approached himself, and may be criticizing an earlier form of his own doctrines. We may observe (1) that he professes only to give us a few opinions out of many which were at that time current in Greece; (2) that he nowhere alludes to the ethical teaching of the Cynics—unless the argument in the Protagoras, that the virtues are one and not many, may be supposed to contain a reference to their views, as well as to those of Socrates; and unless they are the school alluded to in the Philebus, which is described as 'being very skilful in physics, and as maintaining pleasure to be the absence of pain.' That Antisthenes wrote a book called 'Physicus,' is hardly a sufficient reason for describing them as skilful in physics, which appear to have been very alien to the tendency of the Cynics.

The Idealism of the fourth century before Christ in Greece, as in other ages and countries, seems to have provoked a reaction towards Materialism. The maintainers of this doctrine are described in the Theaetetus as obstinate persons who will believe in nothing which they cannot hold in their hands, and in the Sophist as incapable of argument. They are probably the same who are said in the Tenth Book of the Laws to attribute the course of events to nature, art, and chance. Who they were, we have no means of determining except from Plato's description of them. His silence respecting the Atomists might lead us to suppose that here we have a trace of them. But the Atomists were not Materialists in the grosser sense of the term, nor were they incapable of reasoning; and Plato would hardly have described a great genius like Democritus in the disdainful terms which he uses of the Materialists. Upon the whole, we must infer that the persons here spoken of are unknown to us, like the many other writers and talkers at Athens and elsewhere, of whose endless activity of mind Aristotle in his Metaphysics has preserved an anonymous memorial.

V. The Sophist is the sequel of the Theaetetus, and is connected with the Parmenides by a direct allusion (compare Introductions to Theaetetus and Parmenides). In the Theaetetus we sought to discover the nature of knowledge and false opinion. But the nature of false opinion seemed impenetrable; for we were unable to understand how there could be any reality in Not-being. In the Sophist the question is taken up again; the nature of Not-being is detected, and there is no longer any metaphysical impediment in the way of admitting the possibility of falsehood. To the Parmenides, the Sophist stands in a less defined and more remote relation. There human thought is in process of disorganization; no absurdity or inconsistency is too great to be elicited from the analysis of the simple ideas of Unity or Being. In the Sophist the same contradictions are pursued to a certain extent, but only with a view to their resolution. The aim of the dialogue is to show how the few elemental conceptions of the human mind admit of a natural connexion in thought and speech, which Megarian or other sophistry vainly attempts to deny.