Reflecting on the seeming tautology, 'What exists exists,' Jacques Maritain writes,
This is no tautology, it implies an entire metaphysics. What is posited outside its causes exercises an activity, an energy which is existence itself. To exist is to maintain oneself and to be maintained outside nothingness; esse is an act, a perfection, indeed the final perfection, a splendid flower in which objects affirm themselves. (A Preface to Metaphysics, Sheed and Ward, 1939, pp. 93-94)
This is the sort of writing, florid and French, that drives analytic philosophers crazy and moves them to mockery. But I think Maritain is here expressing an important insight. Let me see if I can explain it with as little reliance as possible on Maritain's Thomistic machinery.
1. A tautology is a logical truth, a truth true in virtue of its logical form alone. Now it certainly does seem that 'What exists exists' is true in virtue of its logical form alone. Write it like this: For any x, if x exists, then x exists. By Universal Instantiation, we get if a exists, then a exists, which is of the form, if p then p, which is equivalent to p or not-p, which is the Law of Excluded Middle.
2. On the other hand, it has been clear for a long time that 'exist(s)' is no ordinary predicate. To say of an item that it exists is not to characterize it or classify it. Existence is not a classificatory concept. It doesn't partition neutral items into two classes, the existent ones and the nonexistent ones. Pace Meinong, there are no nonexistent items. And existence certainly does not partition existing items into two classes, the existing and the nonexisting. When I say of a thing that it exists I am saying that it is not nothing. I am not saying that it is F or G, but that it is. I am pointing to its sheer being or existence.
3. The same goes for 'What exists, exists.' Although it can be used to express a tautology, it can also be used non-tautologically. Used non-tautologically, it does not say that that-which-exists is that-which-exists; it says that that-which-exists exists. In other words, it does not say, tautologically, that beings are beings; it says, non-tautologically, that beings are.
4. Somewhere in The Enneads Plotinus writes, "It is by the One that all being are beings." But there would be no need to drag The One into the picture if 'all beings are beings' is a tautology. Tautologies do not need truth-makers. Plotinus' point, of course, is that it is by the One that all beings are. They are in virtue of the One; their Being derives from the One. Whether or not that it true, we understand what is being said and we understand that 'all beings are being' is not a tautology.
5. Metaphysics targets the existence of that-which-exists, the Being of beings, the esse of entia, das Sein des Seienden. Thus metaphysics presupposes a difference between existence and the existent. But existence is "odious to the logician" as George Santayana once observed. (Scepticism and Animal Faith, Dover, 1955, p. 48, orig. publ. 1923.) And so the logician will try to knock the wind out of the metaphysical sails by trying to accommodate the difference between existence and what exists in some such aseptic fashion as the following:
x exists =df for some y, y = x.
Accordingly, existence is identity-with-something-or-other. 'Exists' as a load-bearing predicate gets replaced by some purely logical machinery: the particular quantifer, a bound variable, the identity sign, and a free variable. Existence for the logician is a 'thin' topic. Thin to the point of being anorexic. It is just logical bones bare of metaphysical meat.
6. Well, why not be a thin theorist? I have written a lot on this topic, so now I will be very brief. While it is of course true that everything that exists is identical to something, namely, itself, this presupposes that the things in question exist in a sense that cannot be captured by the above definition. Another way of putting the point is that the above definition is circular. For it amounts to
x exists =df for some y that exists, y = x.
If I want to know what it is for something to exist, I learn nothing by being told that it is identical to something that exists, although that is of course true.
7. Getting back to Maritain, he is right as against the thin theorists: existence is a metaphysically weighty topic. 'What exists exists' can be given a non-tautological reading. But on the thin theory, it could only amount to the tautological 'What is identical to something is identical to something.' But whether existence is a perfection, or indeed the final perfection, or rather the opposite, as Santayana and Sartre would maintain, is a further question.
8. Unfortunately, no resolute thin theorist will be persuaded by anything I or anyone says to abandon his theory. All my dialectic can do is lead the reader to a point where he either gets it or he doesn't, where he either sees it, or he doesn't. You can lead a horse to water, but you can't make him drink.
It's a bit like arguments over religion. If you think that religion is nothing but a tissue of childish superstitions, will I ever be able to convince you otherwise? No. For it is not a matter of analytical intelligence, but of insight, or rather, in your case a lack of insight.