Philosophers in the Aristotelian tradition have arguably offered the best account of metaphysics. What Thomas, Averroes, Maimonides and others, were able to champion much later after Aristotle was the distinction between essence and existence. It was Parmenides who stated that nothing can differ. All must be one, since things either differ by being or non-being. But, to be distinct from being by being is to be non-being, and to be distinct from being by non-being is to be not-distinct at all. Parmenides concluded that all is one.
What Aristotelians were able to accomplish is: 1) a demonstration that things differ; and 2) that Pure Being is needed to ground each essence that isn't identical to its existence.
1. Something exists.
This is undoubtedly true. In order to doubt that I exist, I must first exist in order to doubt it.
2. Something exists only if there is such a thing as existence.
Again, this doesn't need any extensive argumentation. If there is no existence, then nothing can exist.
3. Existence is Pure Being.
Let's first define a couple of terms:
essence: what something is
existence: that something is (its being, actuality, or reality)
For example, what is the difference between a real dollar bill and an imaginary one? The essence of each is the same (a dollar bill has a specific shape, color, smell, etc.), so obviously the difference is that the real dollar bill has existence!
Now, if the essence of a dollar bill is a dollar bill, then the essence of existence must be existence itself. "Pure Being" or "Pure Existence" are the common philosophical terms for existence itself, so premise (3) is true by definition. I have chosen the term "Pure Being" since that is the most used term by metaphysicians, in addition to the fact that this way we may avoid a linguistic redundancy.
4. Therefore, Pure Being exists.
(4) follows from (1)-(3). So, what else can be known about Pure Being? At least five things:
First, Pure Being must have necessary existence. Imagine a state of affairs in which nothing exists. Is such a state of affairs possible? If the state of affairs exists, then something exists. If the state of affairs does not exist, then it's meaningless to talk about a state of affairs in which nothing exists. In short, it is contradictory to posit an existing state of affairs in which nothing exists. Finally, if something exists, then Pure Being exists, as was shown above.
Secondly, Pure Being must be unique. If there were more than one Pure Being, then there would be distinctions between them. However, to be distinct from existence is to be non-existence. Therefore, what is fully existence (Pure Being) must be one and not many.
Third, Pure Being must be omnipresent (all-present). In every existing place, something exists. Yet, something can only exist if it is grounded in Pure Being. Hence, Pure Being exists everywhere.
Fourth, Pure Being is eternal. We have already seen that Pure Being has necessary existence. Now, what exists necessarily cannot at any time fail to exist; otherwise, it would be contingent and not-necessary. Because Pure Being is necessary, it must exist at all times and therefore must be eternal.*
Fifth, Pure Being must be distinct from everything else that exists. Pure Being has an essence that is identical to its existence, since the essence of existence is existence itself. However, every other existing thing has an essence distinct from its existence (a dollar bill does not have necessary existence, but may or may not exist).
What all of this implies is that while things may not differ by existence, they can (and do) differ by essence. Something either exists or it doesn't, but things may differ by what type of things they are.
This is also the basic metaphysical proof of God's existence found in St. Thomas Aquinas' De Ente et Essentia ("On Being and Essence"). It's not entirely original with Thomas, but I think he puts it the best way. So far, this is the easiest summary I've been able to come up with.
You can be sure of at least three things, then: 1) I exist; 2) God exists; and 3) I am not God.**
*Whether Pure Being is timeless or omnitemporal is up for debate.
**Feel free to read "I" in the first-person, rather than in the sense that "Doug Benscoter exists."