You could say that Being itself does not really exist since it is a universal. Only the individual really exists. (It is this aspect of Aristotle which can be made to fit in with Plato perfectly by means of Plotinus. For in the scheme of Plotinus, the One is above existence. He emanates existence. So you could say that Emanation is only the beginning of existence and does not really get to real existence until one comes down to this world.) And that is how he deals with the problem of Parmenides. ["What is, must be. What is not, can't be." This puts a block against all existing things that change. The block is that they can't exist.]
For to Aristotle every category and genus has a sub-category until you get to the individual thing. ["There is a hierarchy of universals and particulars" Stanford Encyclopedia of philosophy] And only that individual has real existence. The genus is secondary. "Things that exist" is the widest genus.
[The idea here is that to Aristotle, universals don't have real existence unless and until they are attached to an individual. (Later I discovered Abelard said the same thing.) So since Being itself or the category of "all things that exist" is a universal, so it does not have real existence until it gets condensed into individual things.
So Aristotle really does deal with Parmenides. His answer is individual things exist. But this does not explain the question that then you are saying individual things come into existence, and that can't be so to Parmenides. Even if you say the individual thing is containing substance and form together, there is still change. Even if matter always existed as Aristotle holds, there still is change. So how does this answer anything???]
( I think it is the same as Plato just backwards. For to Plato the closer up you get to the One the more things are real. To Aristotle the closer you get to the individual thing the more aspects of it get real. The real thing is the combination of them all -- the essence and the substance and the form. But the problem with this is that it changes. So you could say only the One has the real combination of substance form and essence. This might be a way of getting Aristotle to agree with Plato.
Or this might mean that Aristotle essentially comes down to the Monads infinitely small particles that don't change.)
 Perhaps the largest category "all Being" to Aristotle does exist and it is the ground of being of Spinoza, i.e. pure substance.
 For Aristotle to answer Parmenides (what is must be, what is not can't be so there can't be any change and there can only be one existing Substance); he could say only self sustaining existence must exist and that is God. Everything else is dependent existence.
 To Aristotle, God has no substance or matter. For in his view, all creation is divided into matter and form. But it is in a hierarchy until you get to the highest form with no matter -- the First Mover. This simple view point is would imagine probably the Rambam's also. [With Plato and kabalah there is not something from nothing. If one believes in creation is something from nothing he is forced to the Aristotelian Maimonides point of view.] You can't go with straight Aristotle, because to him matter is eternal. With Ibn Rushd and the Rambam you get Creation.
 Aristotle does not have a problem from things coming into existence from non existence. He just requires a cause for it. So he really does not care much about the point of Parmenides in the first place.
 It occurs to me that to Aristotle chance can be a cause. And since matter and waves are really just probability waves until there is some observer (like God) so in fact he is right that probability is the cause of everything we see.
 7:07 AM 7/4/2008. Aristotle says if one realizes that one element is matter and the other form, one is potential and the other actual-- the one will understand that substance is a unity even though it is a composite. The whole is something more than the parts.
But my point here that this does not work for universals (secondary substance).
This might be considered a proof for Abelard (who said that there is no such thing as universals).
Yet Aristotle himself comes up with idea here (H:6 intelligible matter--. the point is in physical matter there can’t be a perfect circle. But in intelligible matter there is. It is the mater of mathematics.)