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Existence is an ontological topic par excellence. In Anglo-American philosophy (this article will have to be augmented with summaries of work in other traditions), probably the most widely-asked question about it is what sort of concept it is, or what function it serves in languages, both natural and formal. Another significant topic, related to the first, but somewhat less discussed, is whether 'existence' or 'exists' can be analyzed or defined or otherwise explicated, and if so, what the explication might be. Perhaps the most common definition/usage of the concept, is to be in the current moment in time, to be in the present, rather than the past or the future. (For further discussion, see the existence of physical objects.)
Hume and Kant, among others, are each well-known for their shared view that the claim that a thing exists, when added to our notion of a thing, does not add anything to the concept. For example, if we form a complete notion of Moses, and superadd to that notion the claim that Moses existed, we are not adding anything to the notion of Moses. Frege and Russell, among many others, for similar reasons are well-known for their view that 'exists' is not a (logical) predicate, or more precisely, not a first-order predicate, or that existence is not a property. This has become the dominant but not the universal view in twentieth-century and contemporary Anglo-American philosophy. But, as G. E. Moore pointed out in an early essay, it is a matter of some difficulty to say what exactly this view amounts to. In recent times, Miller re-visited and created a (now widely accepted) formal demonstration of how existence is a predicate, since it individuates its subject by being its bounds. More can be seen on this in the Stanford essay at the bottom.The words (and concepts) 'existence' and ' being' are treated in slightly different ways in Western philosophy. Aristotle pointed out that there are various ways in which a thing can "be" and inaugurated ontology as a field with his notion that there are categories of being, such as substance, attribute, and acting-upon. Similar claims, however, are not as often made on behalf of existence. That is, contemporary philosophers at least are wont to treat existence as a univocal, unambiguousA word, phrase, sentence, or other communication is called ambiguous if it can be reasonably interpreted in more than one way. The simplest case is a single word with more than one sense: The word "bank", for example, can mean "financial institution", "ed concept, as if the only sense of 'existence', or the only sort of existence worth talking about, were the existence of physical objects. Consequently, some discussions of existence have an unclear bearing on, for example, the sense in which numbers, possibilities, and properties exist (or might be thought to exist).
Even if the ambiguity of 'exists' is sometimes overlooked, oddly enough, the ambiguity of 'does not exist' is not. That is, ontologists are fond of pointing out that there are various ways in which things can be nonexistentNonexistence is sometimes mentioned in discussions of the meaning, or analyzability, of " existence. It is frequently pointed out on this topic that there are a variety of senses in which something can fail to exist (see the following list, as just an exa.
Though often not discussed under the heading of existence, disputes among realismRealism is commonly defined as a concern for fact or reality and rejection of the impractical and visionary. However, the term realism is used, with varying meanings, in several of the liberal arts; particularly painting, literature, and philosophy. It is, phenomenalismIn the philosophy of perception, phenomenalism is the view that physical objects, properties, events (whatever is physical) are reducible to mental objects, properties, events. Ultimately, only mental objects, properties, events, exist. In particular, we, physicalismSee also the old text of this article Physicalism/Larry's text. Physicalism is the metaphysical position that everything is physical; that is, that there are no kinds of things other than physical things. Likewise, physicalism about the mental is a positi, and various other metaphysical views concern what might be called the criteria for existence. For example, phenomenalism, generally speaking, is the view that everything that exists is mental. Most phenomenalists would want to deny that this claim is a definition of 'exists'; if phenomenalism were treated as a definition of 'exists', then others might accuse the view of trying to be "true by definition." Accordingly, it might be dismissed as a trivial exercise in redefining the ordinary concept of existence, which is, perhaps, of little interest to anyone. Exactly what relation, however, definitions (or analyses, or explications, etc.) and criteria have is an interesting and vexed question. See definitions vs. criteria .