Editor's note: I changed a couple of words and added a few line breaks to make the text more readable and comprehensible, since this was taken out of an interview. It took a while for me to transcribe all of this, so here it is. This really was made for myself, to understand everything he was saying. Enjoy.
Once you've taken a look at that, at all a reality, and what is common to all that, you've hit this: that everything is each thing that's real. Again, you have to say of it always two things: this is real, it exists and that exists, and the other thing exist. You have to say something common of all of them is, or [the word] "exists", or whatever term that you want to use in a language, or whatever way they express existence. That's something common that must be said of every single thing that's real; otherwise, it's despair. So, that's a statement that you make of everyone, every being compared to every other being. You have to get a synoptic vision, any being like my own self compared to every other being. I have to make two statements about it: it is but it is not the others. It is, but it is such, so it is not that. So, it both it's like all things that exist; it's unlike everything else in that it is this not that.
Now, those two aspects are just common, and to this, which is unique to each one, those are two attributes that must be said of everything real. At least when you have more than one, if you just had one, you could just say it is. That's all. But as soon as you have more than one, you have to say it is and this is not that. Okay, then when you examine those two, you discover they are really irreducible to each other. You can't reduce them because they are contraries, not contradictions because one negates something had to say you couldn't have that things are common like each other by the identical property by which they differ that doesn't make sense. It's not because they're different that they're like. That doesn't make sense [because they are different] they're so, otherwise you would say if John Smith exists precisely because is just John Smith and everything that exists would have to be John Smith, that's what makes him exist, or if he's exists because he's John Smith or he's John Smith then everything would have to have that same [being as John Smith] but that's not true because others have existence and they're not John Smith, so these must be objectively irreducible aspects: the "this"ness and then the existence which it shares, because the existence is shared by others so it can't be just unique here; no, it's also shared by all things alike.
So, then you have to go down into the engine-room of beings and say that in each being, except possibly one, there must be some kind of internal structure where there's a "this" that makes it this particular being. This internal structure enables it, gives it help to share existence, and have that in common with all beings. Therefore, you have an internal structure that makes things unlike and that which makes things like. You call one the act of existence and the other you call essence.
When you try to relate those two together, could you conceive of existence as a minimum common level? Adding all the different essences for this and that won't work because anything you add, unless it already exists, is adding nothing. Therefore, existence can't be reduced to a minimum. If you add nothing to it, there will be no addition. Existence must be an all-enveloping maximum, and then you'd have to distinguish things by subtracting, so you can say, "this is but only so far." By putting existence as the basic all-enveloping perfection, and then essence has been a limit, progressive limits, then you can work it out that everything has existence and yet it's distinct. So, you couldn't do it by adding on to existence because you can't add anything to what doesn't exist, it would just be mental. So, existence becomes a maximum and then you subtract. For instance, if you imagine God thinking of the universe, He might envision images of Himself: this is like me, but it is not eternal, infinite, and so forth. Like that, all the different beings would exist with a set of minuses, more or less minuses. "Existence, but." The only one that would be existence unqualified would be the divine name "I AM." Period. No buts and no qualifications, just "I AM." You said that you also say everything, although you don't know it yet.
Now, how do you arrive at the necessity of there's one being that is pure act of existence? Well, you proceed through the analysis of all the others. Except perhaps one, all would have to be finite, because whenever you distinguish beings, you imply that this being is not that being. Necessarily, one of the two must lack something that the other has; otherwise, they would just merge into identity. Whenever you say "this is not that," one of the two must lack something. So, it is impossible to have more than one purely infinite being. So, every being in the universe, except perhaps one, must be finite, to distinguish it from all the others.
Now, could everything have limited existence? That's not gonna work, because then you owe one of the basic ascents to God: that no finite being can be self-sufficient. Any finite being cannot explain why there is only this much existence and no more. If it were self-sufficient, it would be the ultimate source of existence, and then it wouldn't make sense for it to give existence to itself in a limited way. As soon as it is limited, it means it cannot explain why it exists in this limited way rather than in some other possible way. Something external would be needed to determine its limitations; it could not choose its own limited mode of existence. Therefore, the limitation indicates that there is something beyond it. Consequently, if no finite being can be self-explanatory, you must ultimately conclude that the only being that is self-sufficient is the infinite plenitude of existence. Anything less leads you further away.
By starting from the finitude of existence, limited existence, everything would have to be limited, except possibly one that would be distinguished from the others precisely because they are limited. So that there could be one and only one pure act of existence that's unlimited. You say, "Well, there must be, because the finites can't explain themselves." So that's how you'd move it all: the others are participated being, and they can't explain why they have this limited degree. They also can't explain why they all share the same property. If you take any set of beings that share the same real property, you say, "Well then why are they similar?" Saint Thomas says you can't. Whatever you have, given two beings that share the same property, either one causes the other, or they're both caused by a third.
Why is that? It's not because beings are different that they are similar. It is not because they are different that they all share a common existence. It is not because they are many that they got this unity; rather, to understand the commonality among all real entities, one must trace back to a singular source that is the origin of this unity and bestows it upon all others. Without this source, the coexistence of diverse beings and their shared unity cannot be explained. A finite being cannot account for its limitations, nor can it elucidate the universal unity among all beings. Therefore, one must trace back the multiplicity and composition of essence and existence to a single source of pure, unlimited existence, and then you stop.
Source: interview, time-mark 14:49