Do we really have a concept of nothing? It may appear obvious that we do, but I am not so sure. We have the concept of non-existence, but it doesn’t follow that we have the concept of nothing—that is, the concept of nothing at all. When an object goes out of existence it doesn’t dissolve into pure nothingness; it assumes another form. An animal that dies and disintegrates goes out of existence, but what happens is that its material parts lose their erstwhile organization. It is not that the animal vanishes into thin air, leaving no residue. When we get to the basic parts of things the story is the same: either the parts (e.g. electrons) cannot be destroyed, or if they are they simply assume another form (maybe even just pure energy). There is always something left, and conservation ensures that something is never replaced with nothing. In these familiar examples of ceasing to exist we do not employ any concept of nothingness in the strict sense. So we cannot ground the concept of absolute nothingness in the ordinary notion of non-existence.
There is then a question as to what we mean by “nothing” in this very strong sense. What do we mean when we ask why there is something rather than nothing, or try to contemplate the universe before anything existed, or imagine the total annihilation of reality? Have we extended our ordinary concept of non-existence in a direction it cannot tolerate? Have we descended into disguised nonsense? All ordinary attributions of non-existence occur against a background of existence, so what can it mean to speak of absolute nothingness?  Perhaps there is nothing we mean by “nothing” when we use it in the strong metaphysical sense.
Here is an argument for that conclusion. Consider ordinary denials of existence like “No dodos exist” or “No fictional characters exist”: these involve the use of a sortal predicate, which specifies what kind of thing is said not to exist (it is the same with affirmations of existence). However, no such statement could imply that nothing exists, since the only thing whose existence is denied is of a specific kind. Take every sortally qualified denial of existence—the conjunction of these will not imply that nothing whatever exists. In order to reach the concept of pure nothingness we need to use a word like “thing” or “entity” or “being”—no thing or entity or being exists. These words are what are known as “dummy sortals”: they provide no criterion of identity and so cannot be used in statements of number (how many things are there on my desk?). But just as we cannot meaningfully say how many things there are in the world, neither can we say that there is nothing in the world. All that could mean is that nothing falls under the various sortal concepts that can apply to the world; but that doesn’t entail that there is absolutely nothing. Even units of energy, volumes of space, and moments of time are sortally described objects. The concept of there being nothing at all—as distinct from there being no F’s or no G’s—has not yet been given a sense. What is it whose existence is being denied? Anything comes the reply: but what does that mean independently of some sortal to tie it down? The metaphysician who wants to talk about pure unadulterated nothingness must be using some other notion of non-existence—not the notion of specific kinds of object not existing. In order to speak meaningfully of non-existence we need to have a sortal concept in mind, but the bare idea of nothing supplies no such concept—it is simply a placeholder for a sortal concept. We can use the word “nothing” in contexts where a sortal term is presupposed, but in its extended metaphysical use it is a pseudo-sortal: no clear meaning can be attached to it.
When we try to frame the idea of pure nothingness—the absence of all existence—we fail to come up with a genuine concept. But we fail to realize this because we employ a concept of non-existence that applies in contexts that do not envisage complete nothingness, as with the ceasing to exist of an animal or city or mountain. Fictional objects don’t exist, but the minds of their creators do; dodos don’t exist, but their atoms do; persons cease to exist, but their bodies go on: none of these varieties of non-existence add up to the complete absence of everything. We really don’t know what such a state of affairs would consist in. We don’t have a clear and distinct idea of absolute nothingness (no space, no time, no logic, no truth, no empty set). It is not clear that there is a possible state of affairs in which nothing exists—what would it even look like?  It may be true that there is nothing such that itnecessarily exists, but it might yet be true that necessarily something exists—some sortal or other is exemplified. At the least the onus is on the believer in the concept of nothingness to demonstrate how such a concept is possible—that metaphysical uses of “nothing” are more than empty words. Why is there something rather than nothing? Maybe it’s because there is no coherent notion of nothing for the existence of something to negate.
 Sartre’s use of the concept of nothingness in Being and Nothingness is instructive: he holds that the essence of consciousness (the for-itself) is nothingness, but all consciousness is conceived by him as directed to being (the in-itself)—so nothingness can only exist against a background of being. Sartre is not using the concept of absolute nothingness, i.e. the complete absence of all existence; for him, nothingness presuppose being.
 Is there a possible world in which nothing at all exists? But wouldn’t that world itself exist? There is a world in which there is nothing. It is harder to expunge all existence than we suppose. Don’t possibilities exist? In a non-existent universe wouldn’t there exist various possibilities? If they are real possibilities, mustn’t they have being?