The land mass that we call "Kansas" would still exist (unless, of course, the reason that everyone died was that the entire Earth blew up, or something like that). But Kansas the state is a "socially constructed" object (of the sort discussed by John Searle in his book The Construction of Social Reality) and would thus no longer exist. On the other hand, there might still be books and maps that refer to that land area as "Kansas", so extraterrestrials visiting Earth later on (or Earth animals that evolve to replace humans as intelligent residents of a future Earth) might be able to refer to it that way. Most artifacts are like this: The substance they are made of is human- or mind-independent; the use made of them is not---so, a flat tree stump would still be a flat tree stump after all humans die, but if it had been used as a table by some human, it would no longer be a table.

I agree with Jonathan Westphal that there's no simple answer to your question as you pose it.

One (no doubt overly simpleminded) way to approach an answer to the question is to make a list of things that exist and then see if they have any properties in common. But what would you put on this list?

You could think of beginning with a list of all of the kinds of individual things that exist: There are people, there are plants, there are animals, etc. That's going to be a pretty long list, but do these kinds of things really exist? Or is it better to say that only individual things of these kinds exist? So, instead, you should list all the individual people (Adam, Eve, Cain, Abel, ..., you, me), list all the plants (the rose on my desk, the rose on your desk, etc.), list all the animals (my pet cat Bella, your pet dog Fido, etc.). That's going to be an even longer list.

But are there such things? Consider any physical object. We know from physics that it's not really a single thing, but a complex thing consisting of atoms. But we also know from nuclear physics that it's really an even more complex thing that consists of quarks (etc.). So maybe people don't exist, only complexes consisting of quarks and other ultimate subatomic particles. (See Peter Unger's essay "Why There Are No People", Midwest Studies in Philosophy 4 (1979).)

What about artifacts like tables or chairs? Do they exist in addition to such "natural kinds" as people, plants, animals, etc.? They, too, are such complexes of subatomic particles. But even if you want to consider all ordinary, medium-sized spatio-temporal objects as being the kinds of things that exist (instead of just the subatomic particles), artifacts depend on their users for their existence in the sense that something is a table if and only if someone uses it as a table. So, a suitably sturdy cardboard box or a flattened tree trunk could (also) be a table. So, maybe tables don't exist in addition to things like flattened tree trunks.

Instead of making a list (technically called an "ontology") of things, or kinds of things, that exist, you could give a criterion for existence. Two of the most famous are:

(1) Bishop Berkeley's proposal that to be (or to exist) is to be perceived (this is the source of the famous "if a tree falls in a forest and no one hears it, then it doesn't make a sound"): If x exists, then someone perceives x (and that someone might have to be God, just in case no human or animal perceives x but you want to maintain that x exists); and if someone perceives x, then x exists (but then what about dreams and hallucinations?).

(2) Willard Van Orman Quine's proposal that to be (or to exist) is to be the value of a bound variable. In other words, any theory (e.g., a scientific theory expressed in a language for first-order logic with variables and quantifiers) that says "there exists an x such that..." is committed to the existence of such x's. (See Quine's essay "On What There Is".)

For more on these topics, look at the articles in the Stanford Encyclopedia of Philosophy on "Existence", "Metaphysics", and "Logic and Ontology".

Are you content with the question as you have formed it?

What does it mean to exist?

Compare:

What does it mean to garden?

What does it mean to play chess?

What does it mean to help someone in need?

What does it mean to suffer?

What does it mean to kill someone?

Compare the last question with the even more serious, 'What does it really mean to kill someone?' One can perhaps accept this as a question without knowing what its full import is. Perhaps it is a desperate expression of remorse and the wish to make amends. But this cannot be true of 'What is the meaning of gardening?' unless it means something tame, though not therefore unimportant, like, 'How does gardening contribute to your life?' Or perhaps you believe that you have wasted your life gardening, and you are now reflecting on this sad fact, if it is one.

It seems to me, without complete conviction, that the six questions shouldn't be attacked head-on. It really is pretty unclear what they mean and how they work (Cf. obviously, 'What does 'to garden' mean?' asked by someone, speaking in a foreign language perhaps, who doesn't know the English verb. This is very probably a request for a bit of translation. The answer might be, 'Look in the dictionary.' Or it might be to give the foreign language equivalent.)

Perhaps you have something in mind like, 'What is the yield of existence?' (But then this cannot be quite like, 'What is the yield of suffering?') The question becomes, 'What difference, if you like, does my existence on this planet (as opposed to some other?) make?' Or 'What difference does my existence on no matter what planet make?' You might think of the differences it makes to your friends, family, society, or even to God. Or you might simply be reflecting on the fact of your existence, as it were adding up a sum: I did this, I did that, I changed the other thing, and so on.

What difference does existence as a whole make (the existence of everything, everything to which the answer to the question, 'Does it exist?' is affirmative)? Well, all the difference in the world, or all the differences! (Kant didn't realize this.)

Let's make two initial comments to muddy the waters!

1) Accepting some version of a correspondence theory of truth -- e.g. accepting that a true proposition is made true by the existence of a corresponding fact -- doesn't ipso fact make you a realist in your ontology. It will obviously depend what you think about facts! (You could still be an idealist like Berkeley, and suppose the only facts are ultimately those involving God, other spirits, and their ideas.)

2) Accepting that our knowledge of the world depends on a lot of processing of data by the brain using built-in cognitive mechanisms doesn't make you an anti-realist in epistemology. You could still hold that when those processes are working reliably, they successfully give you epistemic access to facts that obtain independently of you and your cognitive mechanisms.

I'd say that talk of a "correspondence theory of truth", "realism about ontology", "conceptual frameworks", and "epistemological anti-realism" is all far too slippery to be very useful. As often in philosophy, it helps to try to frame the issues that might be bugging you in plain terms, without any such jargon, and certainly without any reference to "isms". Sometimes it turns out that it was the jargon that was tangling your thought, or greasing the slide into confusion: but even if you are left with genuine problems, it will at least be clearer what they are!

Sure. For one thing, nature doesn't care about our arbitrary units. Suppose we have a plank of wood that''s exactly a foot long. Now I define a new unit: a schfoot. Anything one foot long is exactly pi schfeet long. Is there any mystery about things being pi schfeet long?

Also -- since we're setting aside issues about quanta and, I assume, the possibility that space is granular, can't we make sense of something changing length continuously? A twig that's a foot long and growing will pass through an uncountable number of irrational lengths on its way from being a foot long to being two feet long.

Here's one quick answer to your first question: What makes a bottle a bottle (more precisely: what makes something a bottle) is whether someone uses it as a bottle, not what it's made of or what its form is. Although the rest of your question is stated in Aristotelian and mediaeval terms of substance and accident, I think that part of your question concerns the nature of artifacts. On that topic, you might take a look at my colleague Randall R. Dipert's Artifacts, Art Works, and Agency (Temple University Press, 1993).

What fun! And indeed, it turns out that Giovanna picked my birthday to show me the error of my ways! :-)

As it turns out, however, I don't think we actually disagree about anything. Giovanna has pointed out, in effect, that folks in her tradition use these terms to mark out a distinction (or, it seems to me, a set of related distinctions) that folks in my tradition would talk about in different language. Needless to say, that's not a comment on the value of either tradition nor on the importance of the problems.

I'll confess that I don't think I have the differences here fully in view yet, but if I have it right, one distinction marked by the existence/being distinction is the difference between individual things that exist, change, have properties, etc., and the background against which the possibility of such existents makes sense. Existence, on this way of speaking, refers to the existent things, and being to this broader metaphysical background. Giovanna points to the problem of understanding how something that's always changing ( afact about existence) can nonetheless be the same thing over time (an issue about being?). This is certainly a question that analytic philosophers continue to find interesting. The kinds of problems that Plato wrestled with -- the temporal world of physical objects (existence) vs. the eternal realm of forms (being?) seems to be in the mic hre as well, and once again, analytic philosophers recognize this problem. And while I will confess to not being very familiar with Heidegger's distinction between being "present-at-hand" -- as an object for investingation -- and being "ready to hand" -- as something we simply use without the need to theorize -- also is a perfectly worthwhile distinction that anbalytic philosophers would talk about in a rather different vocabulary.

But whether any of this has anything to do with the thought that "being is the state of quality of existing" is still not clear to me.

In any case, as I said, lots of related distinctions about here, though different traditions of how to talk about them.

As a philosopher working in the classical European tradition, I see the importance of the distinction between being and existence in very different terms from my colleague, Allen Stairs. His claim that “not everyone sees a distinction here” might be true for a certain segment of the 20th century analytic tradition but is certainly not the case if we look at the history of Western philosophy as a whole, for which the distinction between being and existence has been the central metaphysical question. In fact, I would argue that philosophy was born as the attempt to grapple with this very distinction.

During the late 6th century BC, Heraclitus inaugurated "ontology" (the branch of metaphysics dedicated to the study of being) by focusing on the distinction between being and becoming, which roughly corresponds to that between being and existence. This correspondence was already picked up by Plato who, concerning Heraclitus’s flux doctrine, claimed the following: "Heraclitus, I believe, says that all things go and nothing stays, and comparing existents to the flow of a river, he says you could not step twice into the same river." (Cratylus, 402a). In this quote Plato tries to make sense of one of Heraclitus’s most famous fragments: “Of those stepping into rivers staying the same other and other waters flow.” The fragment brings into focus how a river remains the same over time even though the water flowing through it is never the same.

Already with Heraclitus, the distinction between being and existence invokes the epistemological issue of what can be known and how. Even though Heraclitus does not give a full account of the distinct roles performed by sense experience and reason in relation to knowledge, he certainly suggests what has become their classical alignment. While sense experience can ascertain the “existence” of the world, or its becoming, reason can describe its “being,” assumed as its essential, formal, or theoretical underpinning.

Through many variations, digressions, and permutations, the distinction between being and existence becomes once again the focus of discussion with existentialism. Anticipated by the work of Søren Kierkegaard, during the second half of the 19th century, the question regarding how to define being in relation to existence, and existence in relation to being, takes center stage in Being and Time, Martin Heidegger’s magnum opus. In his “Introduction,” Heidegger claims that the tradition has tried to make sense of the distinction between being and existence by approaching it through the wrong lens. In asking, “What is being?” the tradition has made being into just another existing entity, an impossibility given that being is itself the condition for any entity to exist.

This conclusion brings Heidegger to claim that being and existence are not only irreducible to one another but will remain inscrutable if the way of questioning each of them does not undergo a radical change. Instead of asking “What is being?” Heidegger recommends that we start from looking into the only existing entity for whom being is an all-consuming issue. Thus, in order to access being as such, the human being has essentially to question itself. In this sense, after Kierkegaard, before Sartre, and in sharp contrast to the tradition that interprets human existence in terms of rationality, personhood, or spirituality, Heidegger sees it as the only place where being is brought into view.

You need to be careful to distinguish things and ideas here. Is the question about square circles or about the idea of a square circle?

Compare: there are no such things as unicorns. It would plainly be wrong to say that they "neither exist nor don't exist": unicorns definitely don't exist! But the idea of a unicorn exists and seems coherent enough. Indeed, we are tempted to suppose that there could have been things that fitted the idea.

Likewise, there are no such things as round squares. Like unicorns, round squares definitely do not exist. But this time, though we can frame the idea of a round square -- we grasp that something counts as a round square if it is round and a square! -- the idea is a self-contradictory one in the sense that nothing can possibly count as fitting our idea here.

No doubt there are philosophers who make a distinction of some sort here. (For any sentence of the form "There are philosophers who___" you have a good chance of saying something true...) But one is tempted to ask whether being is the quality or state of existing; not everyone sees a a distinction here.

One possibility: not everything that exists is a being. (For example: water isn't a being.) So one might say that among existing things, being is possessed only by the beings. But now we want to know: what's a being? Is it, for example, an Aristotelian substance — something like a person or an animal? Or is it any physical object? (On that story, my thumb would count as a being, but Aristotle wouldn't agree.) Or is it any "mereological sum" so that not only me and my thumb would count as beings, but so would Charley, whose parts are Dan Quayle, the Empire State Building and the marker at the tip of Key West, Florida? (Mereology is the study of part-whole relations.) The question of what it is for something to be a genuine individual is an interesting one, and it may be the one that lies behind your question, but though I can't say for sure. Perhaps a look at the Stanford Encyclopedia of Philosophy entry on substance would be helpful.