Many hours ago we had a weird exchange, which left me with a vague feeling that I hadn't answered a question or that there was something I meant to come back to. (I've had kind of a confusing day.)
I'm a little confused now, but it's probably my own fault! — Srap Tasmaner
So I've come back thinking I'm now in a frame of mind to figure out what was bothering me.
We were talking about why the
O form (
All As are Bs) doesn't carry existential import, I linked the SEP article about the square again, and then in follow-up you said something that struck me as way wrong, though I wasn't really tuned in just then:
Ex should also be neutral on the matter of existence like its companion Ax. — TheMadFool
And eventually I posted the above and also this:
absolutely the existential quantifier has existential import, and the universal quantifier doesn't -- it's just a kind of souped-up conditional. — Srap Tasmaner
Which, I mean, wtf?
I can see how it happened. You had switched from talking about "universal statements" -- like
All As are Bs -- to universal quantification, like ∀xFx, and I only half realized it. You can see that in the "conditional" comment there, in which I'm clearly still thinking about the
O form even while I'm
typing "universal quantifier"! Didn't this confuse the shit out of you?
So, for the record, these are nothing alike. With modern unary quantification, such as ∃xFx and ∀xFx, you don't have the same question of who has existential import and who doesn't. Variables like x range over a domain of discourse (giddily unspecified in natural language), a bunch of objects that you have already stipulated to "exist" (in whatever sense); all you're doing is figuring out which of them satisfy which predicates.
Since ∃ and ∀ can readily be defined in terms of each other, either they both commit you to the existence of, let's say,
things that are F, or neither does. Quine more or less started this particular way of talking, and he says they do. If nothing satisfies a predicate
F, you can say, 'There's nothing that's
F' or 'There are no
Fs,' etc.
tl;dr: 'Everything is a unicorn' and 'Something is a unicorn' both commit you to there being unicorns. 'Nothing is a unicorn' doesn't. 'Something is not a unicorn' (equivalently, 'Not everything is a unicorn') doesn't, but be careful with this one.