Parmenides is one of the most significant of the pre - Socratic philosophers. His only known work, conventionally titled On Nature, is a poem, which has only survived in fragmentary form. Approximately 160 lines of the poem remain today; reportedly the original text had 3,000 lines. It is known, however, that the work originally divided into three parts:

• A proem (Greek: προοίμιον), which introduced the entire work,
• A section known as "The Way of Truth" (aletheia - ἀλήθεια), and
• A section known as "The Way of Appearance / Opinion" (doxa - δόξα).

The proem is a narrative sequence in which the narrator travels "beyond the beaten paths of mortal men" to receive a revelation from an unnamed goddess (generally thought to be Persephone or Dike) on the nature of reality. Aletheia, an estimated 90% of which has survived, and doxa, most of which no longer exists, are then presented as the spoken revelation of the goddess without any accompanying narrative.

Parmenides attempted to distinguish between the unity of nature and its variety, insisting in the Way of Truth upon the reality of its unity, which is therefore the object of knowledge, and upon the unreality of its variety, which is therefore the object, not of knowledge, but of opinion. In the Way of Opinion he propounded a theory of the world of seeming and its development, pointing out however that, in accordance with the principles already laid down, these cosmological speculations do not pretend to anything more than mere appearance.

In the proem, Parmenides describes the journey of a young man from darkness to light. Carried in a whirling chariot, and attended by the daughters of the Sun, the man reaches a temple sacred to an unnamed goddess (variously identified by the commentators with Nature, Wisdom, or Themis), by whom the rest of the poem is spoken. He must learn all things, she tells him, both truth, which is certain, and human opinions; for, though one cannot rely on human opinions, they represent an aspect of the whole truth.

The section known as "the way of truth" discusses that which is real, which contrasts in some way with the argument of the section called "the way of opinion," which discusses that which is illusory. Under the "way of truth," Parmenides stated that there are two ways of inquiry: that it is, that it is not. He said that the latter argument is never feasible because nothing can not be:

For never shall this prevail, that things that are not are.

There are extremely delicate issues here. In the original Greek the two ways are simply named "that Is" (ὅπως ἐστίν) and "that Not-Is" (ὡς οὐκ ἐστίν)  without the "it" inserted in our English translation. In ancient Greek, which, like many languages in the world, does not always require the presence of a subject for a verb, "is" functions as a grammatically complete sentence. Much debate has been focused on where and what the subject is. The simplest explanation as to why there is no subject here is that Parmenides wishes to express the simple, bare fact of existence in his mystical experience without the ordinary distinctions, just as the Latin "pluit" and the Greek huei (ὕει "rains") mean "it rains"; there is no subject for these impersonal verbs because they express the simple fact of raining without specifying what is doing the raining. This is, for instance, Hermann Fraenkel's thesis. Many scholars still reject this explanation and have produced more complex metaphysical explanations. Since existence is an immediately intuited fact, non - existence is the wrong path because a thing cannot disappear, just as something cannot originate from nothing. In such mystical experience (unio mystica), however, the distinction between subject and object disappears along with the distinctions between objects, in addition to the fact that if nothing cannot be, it cannot be the object of thought either:

Thinking and the thought that it is are the same; for you will not find thought apart from what is, in relation to which it is uttered.

For thought and being are the same.

It is necessary to speak and to think what is; for being is, but nothing is not.

Helplessness guides the wandering thought in their breasts; they are carried along deaf and blind alike, dazed, beasts without judgment, convinced that to be and not to be are the same and not the same, and that the road of all things is a backward - turning one.

Thus, he concluded that "Is" could not have "come into being" because "nothing comes from nothing". Existence is necessarily eternal. That which truly is [x], has always been [x], and was never becoming [x]; that which is becoming [x] was never nothing (Not-[x]), but will never actually be. Parmenides was not struggling to formulate the conservation of mass - energy; he was struggling with the metaphysics of change, which is still a relevant philosophical topic today.

Moreover he argued that movement was impossible because it requires moving into "the void", and Parmenides identified "the void" with nothing, and therefore (by definition) it does not exist. That which does exist is The Parmenidean One, which is timeless, uniform, and unchanging:

How could what is perish? How could it have come to be? For if it came into being, it is not; nor is it if ever it is going to be. Thus coming into being is extinguished, and destruction unknown.

Nor was [it] once, nor will [it] be, since [it] is, now, all together, / One, continuous; for what coming - to - be of it will you seek? / In what way, whence, did [it] grow? Neither from what - is - not shall I allow / You to say or think; for it is not to be said or thought / That [it] is not. And what need could have impelled it to grow / Later or sooner, if it began from nothing? Thus [it] must either be completely or not at all.

[What exists] is now, all at once, one and continuous... Nor is it divisible, since it is all alike; nor is there any more or less of it in one place which might prevent it from holding together, but all is full of what is.

And it is all one to me / Where I am to begin; for I shall return there again.

Parmenides claimed that the truth cannot be known through sensory perception. Only Logos will result in the understanding of the truth of the world. This is because the perception of things or appearances (the doxa) is deceptive. Genesis - and - destruction, as Parmenides emphasizes, is illusory, because the underlying material of which a thing is made will still exist after its destruction. What exists must always exist. And we arrive at the knowledge of this underlying, static, and eternal reality (aletheia) through reasoning, not through sense - perception.

For this view, that That Which Is Not exists, can never predominate. You must debar your thought from this way of search, nor let ordinary experience in its variety force you along this way, (namely, that of allowing) the eye, sightless as it is, and the ear, full of sound, and the tongue, to rule; but (you must) judge by means of the Reason (Logos) the much - contested proof which is expounded by me.

After the exposition of the arche - ἀρχή, i.e. the origin, the necessary part of reality that is understood through reason or logos (that [it] Is), in the next section, the Way of Appearance / Opinion / Seeming, Parmenides proceeds to explain the structure of the becoming cosmos (which is an illusion, of course) that comes from this origin.

The structure of the cosmos is a fundamental binary principle that governs the manifestations of all the particulars: "the aether fire of flame", which is gentle, mild, soft, thin and clear, and self - identical, and the other is "ignorant night", body thick and heavy.

The mortals lay down and decided well to name two forms (i.e., the flaming light and obscure darkness of night), out of which it is necessary not to make one, and in this they are led astray.

The structure of the cosmos then generated is recollected by Aetius:

For Parmenides says that there are circular bands wound round one upon the other, one made of the rare, the other of the dense; and others between these mixed of light and darkness. What surrounds them all is solid like a wall. Beneath it is a fiery band, and what is in the very middle of them all is solid, around which again is a fiery band. The most central of the mixed bands is for them all the origin and cause of motion and becoming, which he also calls steering goddess and keyholder and Justice and Necessity. The air has been separated off from the earth, vaporized by its more violent condensation, and the sun and the circle of the Milky Way are exhalations of fire. The moon is a mixture of both earth and fire. The aether lies around above all else, and beneath it is ranged that fiery part which we call heaven, beneath which are the regions around the earth.

The traditional interpretation of Parmenides' work is that he argued that the every - day perception of reality of the physical world (as described in doxa) is mistaken, and that the reality of the world is 'One Being' (as described in aletheia): an unchanging, ungenerated, indestructible whole. Under the Way of Opinion, Parmenides set out a contrasting but more conventional view of the world, thereby becoming an early exponent of the duality of appearance and reality. For him and his pupils, the phenomena of movement and change are simply appearances of a static, eternal reality.

Parmenides' philosophy is presented in the form of poetry. The philosophy he argued was, he says, given to him by a goddess, though the "mythological" details in Parmenides' poem do not bear any close correspondence to anything known from traditional Greek mythology:

Welcome, youth, who come attended by immortal charioteers and mares which bear you on your journey to our dwelling. For it is no evil fate that has set you to travel on this road, far from the beaten paths of men, but right and justice. It is meet that you learn all things - both the unshakable heart of well - rounded truth and the opinions of mortals in which there is not true belief.

It is with respect to this religious / mystical context that recent generations of scholars such as Alexander P. Mourelatos, Charles H. Kahn, and the controversial Peter Kingsley have begun to call parts of the traditional, rational logical / philosophical interpretation of Parmenides into question (Kingsley in particular stating that Parmenides practiced iatromancy). It has been claimed that previous scholars placed too little emphasis on the apocalyptic context in which Parmenides frames his revelation. As a result, traditional interpretations have put Parmenidean philosophy into a more modern, metaphysical context to which it is not necessarily well suited, which has led to misunderstanding of the true meaning and intention of Parmenides' message. The obscurity and fragmentary state of the text, however, renders almost every claim that can be made about Parmenides extremely contentious, and the traditional interpretation has by no means been abandoned.

Parmenides' considerable influence on the thinking of Plato is undeniable, and in this respect Parmenides has influenced the whole history of Western philosophy, and is often seen as its grandfather. Even Plato himself, in the Sophist, refers to the work of "our Father Parmenides" as something to be taken very seriously and treated with respect. In the Parmenides, the Eleatic philosopher, which may well be Parmenides himself, and Socrates argue about dialectic. In the Theaetetus, Socrates says that Parmenides alone among the wise (Protagoras, Heraclitus, Empedocles, Epicharmus, and Homer) denied that everything is change and motion.

Parmenides is credited with a great deal of influence as the author of an "Eleatic challenge" that determined the course of subsequent philosophers' enquiries. For example, the ideas of Empedocles, Anaxagoras, Leucippus, and Democritus have been seen as in response to Parmenides' arguments and conclusions.

Parmenides made the ontological argument against nothingness, essentially denying the possible existence of a void. According to Aristotle, this led Leucippus to propose the atomic theory, which supposes that everything in the universe is either atoms or voids, specifically to contradict Parmenides' argument. Aristotle himself, proclaimed, in opposition to Leucippus, the dictum horror vacui or "nature abhors a vacuum". Aristotle reasoned that in a complete vacuum, motion would encounter no resistance, and thus infinite speed would be possible, something which Aristotle would not accept.

Erwin Schrödinger identified Parmenides' monad of the "Way of Truth" as being the conscious self in "Nature and the Greeks".

A shadow of Parmenides' ideas can be seen in the physical concept of Block time, which considers existence to consist of past, present, and future, and the flow of time to be illusory. In his critique of this idea, Karl Popper called Einstein "Parmenides".