Many aspects of Parmenides’s thought remain puzzling even when we have collected all the scraps of evidence from his own writings and those of later thinkers who discussed his views. But his immense significance in philosophical terms has never been obscured by the difficulties in the nitty-gritty of interpretation. For one thing, it is obvious that Parmenides throws at us the challenge of whether we should trust our reason or our senses, in circumstances when they seem to conflict.

Parmenides: If the one is not, nothing is. Then, and we may add, whether the one is or is not, the one and the others in relation to themselves and to each other all in every way are and are not and appear and do not appear.

Parmenides: If anyone, with his mind fixed on all these objections and others like them, denies the existence of ideas of things, and does not assume an idea under which each individual thing is classed, he will be quite at a loss, since he denies that the idea of each thing is always the same, and in this way he will utterly destroy the power of carrying on discussion... Then what will become of philosophy? To what can you turn, if these things are unknown?

Parmenides did for science what Plato would later do for morality and aesthetics as well: he alerts us to the fact that opinions are just opinions, and they may differ widely. There may yet be a single truth, which need not be as anyone thought. To search for knowledge is to search for access to the truth, not to collect other people’s opinions, and philosophy conducts its unrelenting search for truth in the steps of Parmenides, by respecting sound and rigorous logical argument rather than the variegated tapestry of unexamined opinions.

The Greek tradition was a complete contrast to that of the Far East. ...the Greeks placed logic at the pinnacle of human thinking. Their sceptical attitude towards the wielding of 'non-being' as some sort of 'something' that could be subject to logical development was exemplified by Parmenides' influential arguments against the concept of empty space. ...He maintained that you can only speak about what is: what is not cannot be thought of, and what cannot be thought of cannot be. ...more unexpected was the further conclusion that time, motion nor change could exist either.

All thinkers then agree in making the contraries principles, both those who describe the All as one and unmoved (for even Parmenides treats hot and cold as principles under the names of fire and earth) and those too who use the rare and the dense. (20)

I shall have to test the theory of my father Parmenides, and contend forcibly that after a fashion not-being is and on the other hand in a sense being is not… For unless these statements are either disproved or accepted, no one who speaks about false words, or false opinion —whether images or likenesses or imitations or appearances— about the arts which have to do with them, can ever help being forced to contradict himself and make himself ridiculous.

Parmenides: It is impossible to conceive of many without one.

The followers of Heraclitus insisted the Immortal Principle was change and motion. But Parmenides' disciple, Zeno, proved through a series of paradoxes that any perception of motion and change is illusory. Reality had to be motionless. The resolution of the arguments of the Cosmologists came from a new direction entirely, from a group Phædrus seemed to feel were early humanists. They were teachers, but what they sought to teach was not principles, but beliefs of men. Their object was not any single absolute truth, but the improvement of men. All principles, all truths, are relative, they said. "Man is the measure of all things." These were the famous teachers of "wisdom," the Sophists of ancient Greece.

...classic philosophy maintained that change, and consequently time, are marks of inferior reality, holding that true and ultimate reality is immutable and eternal. Human reasons, all too human, have given birth to the idea that over and beyond the lower realm of things that shift like the sands on the seashore there is the kingdom of the unchanging, of the complete, the perfect. The grounds for the belief are couched in the technical language of philosophy, but the grounds for the cause is the heart's desire for surcease from change, struggle, and uncertainty. The eternal and immutable is the consummation of mortal man's quest for certainty.

Socrates: I think the most likely view is, that these ideas exist in nature as patterns, and the other things resemble them and are imitations of them; their participation in ideas is assimilation to them, that and nothing else.
Parmenides: It is impossible that anything be like the idea, or the idea like anything; for if they are alike, some further idea, in addition to the first, will always appear, and if that is like anything, still another, and a new idea will always be arising, if the idea is like that which partakes of it.

The thinking or figurate conception which has before it only a specific, determinate being must be referred back to the [...] beginning of the science made by Parmenides who purified and elevated his own figurate conception, and so, too, that of posterity, to pure thought, to being as such and thereby created the element of the science. What is the first in the science had of necessity to show itself historically as the first. And we must regard the Eleatic One or being as the first step in the knowledge of thought.

Parmenides: Whatever the subject of your hypothesis, if you suppose that it is or is not, or that it experiences any other affection, you must consider what happens to it and to any other particular things you may choose, and to a greater number and to all in the same way; and you must consider other things in relation to themselves and to anything else you may choose in any instance, whether you suppose that the subject of your hypothesis exists or does not exist, if you are to train yourself completely to see the truth perfectly.

The view of things [called Pantheism] … — that all plurality is only apparent, that in the endless series of individuals, passing simultaneously and successively into and out of life, generation after generation, age after age, there is but one and the same entity really existing, which is present and identical in all alike; — this theory … may be carried back to the remotest antiquity. It is the alpha and omega of the oldest book in the world, the sacred Vedas, whose dogmatic part, or rather esoteric teaching, is found in the Upanishads. There, in almost every page this profound doctrine lies enshrined; with tireless repetition, in countless adaptations, by many varied parables and similes it is expounded and inculcated. That such was, moreover, the fount whence Pythagoras drew his wisdom, cannot be doubted … That it formed practically the central point in the whole philosophy of the Eleatic School, is likewise a familiar fact. Later on, the New Platonists were steeped in the same … In the ninth century we find it unexpectedly appearing in Europe. It kindles the spirit of no less a divine than Johannes Scotus Erigena, who endeavours to clothe it with the forms and terminology of the Christian religion. Among the Mohammedans we detect it again in the rapt mysticism of the Sufi. In the West Giordano Bruno cannot resist the impulse to utter it aloud; but his reward is a death of shame and torture. And at the same time we find the Christian Mystics losing themselves in it, against their own will and intention, whenever and wherever we read of them! Spinoza's name is identified with it.