The essential weakness of the Eleatic way of thinking was not glaringly apparent, though implicit, in the utterance of Xenophanes. This consisted in the unreconciled antithesis between the world of unity, of true being, of rational thought, and the world of multiplicity, of phenomenal appearance, of opinion. By pushing the tenets of his master to their logical conclusions, and by exchanging theological for metaphysical phraseology, Parmenides, the greatest teacher of the school, exposed the fatal insufficiency of Eleatic dualism. At the same time he achieved an ever-memorable triumph in philosophy by forcing the problem of essential reality upon the earliest Greek speculators, and by defining the battle-ground of future ontological controversy.

Parmenides, a native of Elea, who flourished about the year 503 B.C., enjoyed a reputation in his native city scarcely inferior to that of Pythagoras at Crotona, of Empedocles at Acragas, or of Solon at Athens. Speusippus, quoted by Diogenes Laertius, asserts that the magistrates of Elea were yearly sworn to observe the laws enacted by Parmenides. Cebes talks about a "Pythagorean or Parmenidean mode of life," as if the austere ascesis of the Samian philosopher had been adopted or imitated by the Eleatic. Indeed, there is good reason to suppose that Parmenides held intercourse with members of the Pythagorean sect, his neighbors in the south of Italy. Diogenes Laertius relates that he was united in the bonds of closest friendship to Ameinias and Diochætes, two Pythagoreans. Of these the latter was a poor man, but excellent in breeding and in character; Parmenides so loved him and respected him that, when he died, he dedicated a hero's chapel to his memory. The philosophers of this period in Greece, as might be proved abundantly, were no mere students but men of action and political importance. Their reputation for superior wisdom caused them to be consulted in affairs of state, and to be deferred to in matters of constitutional legislation. Some of them, like Thales, Anaximander, and Empedocles, were employed on works of public utility. Others, like Pythagoras, remodelled the society of cities, or, like Anaxagoras, through their influence with public men like Pericles, raised the tone of politics around them. All of them devoted a large portion of their time and attention to the study of public questions. It was this kind of prestige, we may conjecture, which, in the next phase of Greek thought, threw so much power into the hands of sophists, and which finally encouraged Plato in his theory that those states would be best governed where the sages were the rulers.

Of Parmenides himself some precious notices have been preserved by Plato. It appears that the great Eleatic teacher visited Athens in his old age. Socrates was a young man at the period of this visit; and Plato, whether inventing an occasion for their meeting or relying on actual tradition, brings them into conversation. In the prelude to the dialogue Parmenides, we read:[45]

"He told us that Pythodorus had described to him the appearance of Parmenides and Zeno; they came to Athens, he said, at the great Panathenæa; the former was, at the time of his visit, about sixty-five years old, very white with age, but well-favored. Zeno was nearly forty years of age, of a noble figure and fair aspect; and in the days of his youth he was reported to have been beloved of Parmenides. He said that they lodged with Pythodorus in the Ceramicus, outside the wall, whither Socrates and others came to see them; they wanted to hear some writings of Zeno, which had been brought to Athens by them for the first time. He said that Socrates was then very young, and that Zeno read them to him in the absence of Parmenides, and had nearly finished when Pythodorus entered, and with him Parmenides and Aristoteles, who was afterwards one of the Thirty; there was not much more to hear, and Pythodorus had heard Zeno repeat them before."

The Theætetus contains another allusion to Parmenides, which proves in what reverence the old philosopher was held by Socrates:

"My reason is that I have a kind of reverence, not so much for Melissus and the others, who say that 'all is one and at rest,' as for the great leader himself, Parmenides, venerable and awful, as in Homeric language he may be called—him I should be ashamed to approach in a spirit unworthy of him. I met him when he was an old man and I was a mere youth, and he appeared to me to have a glorious depth of mind. And I am afraid that we may not understand his language, and may fall short even more of his meaning."

Finally, in the Sophistes a passing allusion to the same event is put into the mouth of Socrates: "I remember hearing Parmenides use the latter of the two methods, when I was a young man and he was far advanced in years, in a very noble discussion." These notices of the Eleatic sage, we feel, are not in any sense accidental. Plato has introduced them in important moments of his three most studied dialogues upon those very points which occupied the mind of Parmenides, and by the elaboration of which he made his greatest contribution to philosophy. The problems of knowledge and of the relation of the phenomenal universe to real existence were for the first time methodically treated in the school of Elea. Their solution in the theory of Ideas was the main object of Plato's philosophical activity.

The unity asserted by Xenophanes gave its motto to the Eleatic school; ἓν τὰ πάντα became their watchword. Parmenides, however, abstracted from this unity all theological attributes. Plain existence, obtained apparently by divesting thought of all qualifications derived from sensation and imagination, and regarding it in primitive and abstract nakedness or nothingness, was the only positive condition which he left to the principle of Being; and though he seems to have identified this Being with Thought, we must be careful not to be misled by modern analogies into fancying that his ἀρχή involved a purely intellectual idealism. Nor, again, can we regard it as the totality of things presented to the senses; the most earnest polemic of the philosopher is directed against this view. The Unity, the Being, of Parmenides, was in truth the barest metaphysical abstraction, deduced, we are tempted to believe, in the first instance from a simple observation of language, and yet, when formed, not wholly purged from corporeity. Being is proved by the word ἐστί. The singular number indicates the unity of the subject; the present tense proves its eternity, for it neither asserts a has been nor a will be, but an everlasting is. Its antithesis not-Being is impossible and inconceivable; οὐκ ἐστί. Completing his conception of Being as the sole reality, and carrying out the arguments attributed by Aristotle to his master,[46] Parmenides shows that the eternal One is indivisible, immovable, continuous, homogeneous, absolutely self-identical, beyond the reach of birth, or change, or dissolution. Furthermore, it is finite and spheroid. In rounding and completing his notion of the Unity of Being, Parmenides seems at this point to have passed into the region of geometrical abstractions. The sphere of mathematics requires to be circumscribed by a superficies equidistant at all points from the centre. These conditions of perfection Parmenides attributed to Being, forgetting that the finite sphere thus conceived by him implied, by a necessity of human thought, a beyond against which it should be defined. At the same time, this geometrical analogy prevents us from assuming that the further identification of Being with Thought excluded a concrete and almost material conception of the Ens.

As opposed to this unique ἀρχή, the sole and universal reality, which can only be apprehended by the reason, and which is eternally and continuously One, Parmenides places the totality of phenomena, multiplex, diverse, subject to birth, change, division, dissolution, motion. These, he asserts, are non-existent, the illusions of the senses, mere names, the vague and unreal dreamworld of impotent mortals. Not having advanced in his analysis of thought beyond the first category of Being, he felt obliged to abandon the multiplicity of things as hopeless and unthinkable. Yet he cannot deny their phenomenal existence; there they are, deceiving the sage and the simple man alike: experience asserts them; language and the opinion of humanity take them for granted as realities. Parmenides feels bound to offer an explanation of this cosmos of illusion, this many-formed and many-colored mirage. His teaching consequently contains a paradox deeply embedded in its very substance. Having first expounded the law of absolute truth, he proceeds to render a grave and meditated account of error. Having demonstrated the sole existence of abstract Being, he turns a page and begins to discourse, like any physicist of his age in Greece, concerning Light and Night, Hot and Cold, Fire and Earth, Active and Passive, Male and Female, Rare and Dense. By a singular irony of fate it was precisely for this portion of his teaching that he received the praise of Bacon in the Novum Organum. To connect the doctrine of Being, τὰ πρὸς ἀλήθειαν, and the doctrine of Appearance, τὰ πρὸς δόξαν, was beyond his power. It was what Plato afterwards attempted in his theory of ideas, and Aristotle in the theory of forms and matter, εἴδη and ὕλη. Parmenides himself seems to have regarded man as a part of the cosmos, subject to its phantasmagoric changes and illusions, yet capable of comprehending that, while the substratum of Being is alone immutable, real, and one, all else is shifting, non-existent, and many. Neglect, he says, the object of sense, the plurality of things obedient to change, and you will arrive at the object of reason, the unity that alters not and can be only apprehended by thought. Yet, while on the one hand he did not disdain to theorize the universe of sense, so, on the other hand, as already hinted, he had not arrived at the point of abstracting corporeity from Being. To do this from his point of view was indeed impossible. Having posited pure Being as the sole reality, he was obliged to form a figurative presentation of it to his own mind. A new stage had to be accomplished by human thought before the intellect could fairly grapple with the problems nakedly and paradoxically propounded by the sage of Elea.