Non-being looks like it cannot be. Being is always positive, never negative. It never contains lacks or absences or negations. There are no “negative facts”: actualities with not-ness built into them. Negation belongs with language, mental acts, not with objective reality. There is no such thing as nothingness. Yet negation is real; it’s not like unicorns and phlogiston. It exists. So, it looks like it both exists and doesn’t exist, which is contradictory. It therefore poses a philosophical puzzle (Parmenides first raised this puzzle). Possibility is similar: possibilities apparently exist—we can talk about them, envisage them. But they also appear not be like other real things: they are merely possible, pure potentialities. They don’t belong to the world of actuality. They thus seem to exist and not to exist, which is contradictory. Some seek to avoid the contradiction by distinguishing existence and actuality, but this is a philosophical maneuver not a piece of common sense. Time has being, but it also lacks the usual marks of being: the past does not exist, nor does the future, and the present is just a duration-less point. The being of time is permeated by non-being. Space exists, but it lacks the substantiality of matter; it seems like an absence not a presence. Value seems to be, but not in the way non-value facts are: we can’t encounter it in the world. It is and it isn’t. Free will can seem solidly real and yet it vanishes upon logical examination. Colors are as real as anything, we think, but on reflection they don’t belong to objective reality; they disappear into the mind or the merely imaginary. The self indubitably exists and yet it can’t be found anywhere in the mind (or body). Causation is part of concrete reality, but we can’t have an impression of it. Meaning disappears under examination, but is part of commonsense belief. In all these cases, things seem to have a contradictory nature: to be and not to be. Hence the threat of elimination, denial, reduction. At a pre-reflective level, we take their existence for granted, but we can be quickly led to doubt their existence. In some moods we might affirm their contradictory nature (as with Sartre’s view of consciousness, which “is what it is not and is not what it is”). These things are under existential threat. The verdict of non-existence hangs over them (“The court hereby finds you guilty of non-existence in the first degree”). They seem to straddle being and non-being, being both positively and negatively charged, as it were. Not surprisingly, then, they invite philosophical puzzlement, conceptual unease. They suffer from a kind of existential indeterminacy or uncertainty, as if they can’t make up their mind whether to belong to the realm of being or non-being. And this seems to be part of their philosophical make-up: they are under constant threat of non-existence (all have been denied existence at one time or another). What does not suffer from such a threat? Shapes and sense data don’t: these both have being without the simultaneous presence of non-being. Their being is wholly positive (according to traditional conceptions). They are not thought to have a foot in both camps. Fictional entities are straightforwardly non-existent, shapes and sense data straightforwardly exist, while the items listed hover uneasily between the two. The metaphysician happily appeals to shapes and sense data as a foundation, but is reluctant to go all in with non-being, possibility, value, the self, etc. Thus, we have materialism based on shapes (“extension”) and idealism based on sense data (or “thoughts”). These things unequivocally possess being, without any admixture of non-being, but the entities listed uncomfortably combine being with non-being. We don’t want to base our metaphysics on entities that court contradiction and flirt with non-existence. This seems characteristic of the philosophical landscape: the troubling entities are existentially ambivalent, while the untroubling ones are fully in the realm of being. On the face of it at least: someone might labor to convict shapes and sense data of existential delinquency, and acquit the listed items of their apparent crimes against logic and a robust sense of reality. But the overwhelming impression is that the former items are in good existential standing while the latter are manifestly uncertain of where they belong in the grand scheme of things. All the standard moves in philosophy can be seen as responsive to this dichotomy: ontological favoritism, elimination, reduction, defiant realism, etc. And there is good reason for this to be so, since it is genuinely perplexing how anything could both exist and not exist. Our concept of existence is stretched by these entities; it really is hard to make up our minds about whether they exist or not (Meinong is always a tempting option). In daily life their existence seems assured, but in the study, coolly viewed, they start to look tainted with non-existence (hence various kinds of fictionalism). Philosophy might be seen as a response to existential ambiguity or doubleness.
 I haven’t tried to defend these claims or reply to objections; my aim is simply to list them so as to display a pattern. I think it is clear that there is an intuitive issue in each case. There is something that needs to be resolved, reconciled—a conceptual conflict, an ontological tension.
 Maybe not all of it, but large chunks of it. Does X exist or not or both? On What There Is, and Also Isn’t. Philosophy is a battle with non-existence, or its permanent possibility. Existence and non-existence are never self-evident. The concept itself lacks transparency. Does existence even exist (it isn’t an ordinary property)?