existence, in metaphysics, that which applies neutrally to all and only those things that are real.
Metaphysicians have had a great deal to say about the existence or nonexistence of various things or categories of things, such as God, the soul, a mind-independent or external world, abstract or ideal forms and other universals, possible but not actual objects or worlds, individual essences, and free will. They have had less to say, however, about existence itself—that is, about the content of the concept of existence or about the meaning of the word existence. They have said enough, however, to make possible a taxonomy of theories of existence. Such a taxonomy can be presented as a list of pairs of opposed or contradictory theses about the nature of existence.
1. Some metaphysicians have affirmed, and others have denied, that existence is the same as being. It may seem obvious that “Mountains higher than Mont Blanc exist” and “There are mountains higher than Mont Blanc” are two ways of saying the same thing. But some metaphysicians believe that there are things that do not exist—fictional characters, for example, or the Greco-Roman gods and goddesses. Their position is that, although such things certainly do not exist, the fact that there are such things implies that they have being. If something can “be” without existing, they argue, then existence and being must be distinguished.
2. Some metaphysicians have affirmed, and others have denied, that existence is a barren or empty or trivial concept. The German idealist philosopher G.W.F. Hegel (1770–1831), for example, referred to being—which he did not distinguish from existence—as “the very poorest and most abstract” of all categories.
3. Some metaphysicians have affirmed, and others have denied, that the word exist means the same thing in all its applications. For example, mathematicians habitually speak of the “existence” of abstract, mathematical objects such as numbers or functions. Metaphysicians, as well as philosophers of mathematics, differ on the question of whether existence in such assertions means the same as it does when it is applied to persons and other tangible, visible things.
4. Some metaphysicians have affirmed, and others have denied, that the being (or existence) of one object may be “more perfect,” or “of a higher degree,” than the being (or existence) of another. A classic expression of this idea is the analogy of the divided line, representing a fourfold hierarchy of being, from the Republic of Plato (428/427–348/347 bce): images and shadows participate in being very imperfectly, sensible objects less imperfectly, and “mathematicals” (geometric lines and figures) less imperfectly still. But the eternal, unchangeable forms—and only they—exhibit being perfectly.
5. Some metaphysicians have affirmed, and others have denied, that existence is a property or an attribute of everything that exists. The German Enlightenment philosopher Immanuel Kant (1724–1804), the most famous critic of the thesis, identified it as the fallacy on which the ontological argument for the existence of God depends. Deniers of the thesis have maintained that “existence” statements are only apparently about the things that are their grammatical subjects and so cannot be understood as attributing a certain property to those things. The German logician Gottlob Frege (1848–1925), for example, held that the statement “Horses exist” really means “The number of objects that fall under the concept horse is not zero.”
A theory of existence may be identified with some combination of the theses discussed above. It should be noted, however, that some combinations are inconsistent, or at least apparently so. For example, anyone who accepts Frege’s account of existence seems to be committed to the theses that existence is a trivial concept, that there is no distinction to be made between being and existence, that existence means the same thing in all its applications, and that existence is not something that one thing can exhibit more perfectly or in some higher degree than another. See also ontology.