Aargh, waaaay past my bedtime! Again! So I must be brief. I've heard the slogan (kennethamy quoting Quine?) that to exist is to instantiate (satisfy?) a predicate. (Sorry, KA, if you're out there in limbo, and reading this, and gnashing your teeth at a silly error which you can do nothing to correct!) But I don't understand it. Seems to me that there are all sorts of problems with this. (Also, you didn't respond to my point about the apparent implication of necessary existence.) For one thing, you seem to be defining existence in terms of itself, thus: x exists iff there exists F such that Fx. Is this apparent circularity not real? It also seems to me that in order for the expression Fx to mean anything [the forum doesn't do Quine quasi-quotes, and anyhow I don't know how to use them], because F is not being asserted of the symbol 'x' itself, but rather of whatever x denotes, it must be presupposed that x denotes something, which is surely another way of saying that x exists? So that you would seem to have more than one kind of circularity (unless it's only my tired and addled brain that's going round in circles). And finally, and most immediately (I'm putting these objections in reverse order, for some reason), treating existence as a predicate seems to open you to the subtle fallacies associated with versions of the ontological argument, some of which, at least, Russell's theory of descriptions seems to sort out pretty nicely. (I must admit I haven't studied this stuff properly, but I'm just giving my immediate reactions, which seem natural enough to me. Sorry if, for reasons perhaps obvious to the more informed, they are off-beam.)
In my opinion, the pure concept just is.
It's a Form. This ties into Being and beings, doesn't it? The concept just is.
It is irreducible. Hence the beginning of Hegel's logic.