Parmenides
The greatest of the pre-Socratics, Parmenides of Elea invented the field of metaphysics and started the field of philosophy proper. He elevated philosophy from the mere physical to the speculative, and correctly perceived the true nature of being.
In his poem On Nature, Parmenides notes that there is being (that which is) and nonbeing (that which is not) in the world. Only being is worthwhile to investigate, since “Thou canst not know what is not — that is impossible — nor utter it”.
Parmenides lays down the Law of Non-Contradiction and uses it to refute Heraclitus’ arguments that a thing can exist and not exist. Furthermore, he uses it to prove that being cannot come from nonbeing, since that would imply things that are not becoming things that are. In addition, it is unimaginable what “source could it [nonbeing] have drawn an increase”, and “what need could have made it arise later rather than sooner”.
Being is “complete, immovable, and without end. Nor was it ever, nor will it be; for now it is, all at once, a continuous one.” Being is complete, which is to say it contains “in itself every perfection” (ItP). This is correct, since everything outside of being does not exist, and imperfections lack being, thus being is the perfections. Being is immovable, which is to say it lacks motion, change. This is true, since being cannot become nonbeing (this would violate the Law of Non-Contradiction), and being cannot become more being — it already is being. Being and non-being are contradictories. Being and nonbeing are similar to the statements “being on the moon or not being on the moon” (CdF). You are either on the moon or you are not, and there is no intermediate point where you are both on and off the moon. Being is without end, which is to say it is eternal — it never has changed, not has it come out of nonbeing, nor will it come into nonbeing.
“Nor is it divisible, since it is all alike, and there is no more of it in one place than in another, to hinder it from holding together, nor less of it, but everything is full of what is. Wherefore it is wholly continuous; for what is, is in contact with what is.” Parmenides has stated that being is identical to itself (Law of Identity), and therefore there is no being that is more being than other being — everything is being. There is only nonbeing available to keep it apart, but since nonbeing does not exist, it does not matter. Therefore being cannot be divided.
Consequences of Being
And as he contemplated pure being, he perceived that this being is completely one, absolute, immutable, eternal, without becoming, incorruptible, indivisible, whole and entire in its unity, in everything equal to itself, infinite and contain in itself every perfection. But while he thus discovered the attributes of him who is, he refused to admit that any other being could exist, and rejected as a scandal to the reason the being mingled with non-entity, because produced from nothingness, of every creature. (ItP)
Using his conclusions on the nature of being, Parmenides argued that (a) change cannot occur and (b) that the world was an illusion. Since something is identical with itself, if it ceases to be itself by change, it creates something new ex nihilo, which is against both the laws of Identity and Non-Contradiction. Parmenides’ student, Zeno of Elea, expanded on this idea with his famous paradoxes, such as that of the arrow. Zeno posited that for an instantaneous amount of time, the arrow was not actually moving, but staying still1. However, since the whole period of time is composed of these instantaneous moments, motion is impossible.
As Maritain states above, Parmenides denied that sensible reality existed, because it did not possess the attributes of being. Things in reality are neither one (they often have multiplicity); are not immutable (they seem to change); and are definitely not perfect. Thus Parmenides denied the existence of everything other than pure being.
It would not be until Aristotle that these quagmires raised by Parmenides would be resolved; that the subject that endures change would be found; and that reality would be explained as the mingling of being and non-being.
https://en.wikipedia.org/wiki/Zeno%27s_paradoxes#Arrow_paradox