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History

The roots of ontology in ancient philosophy are speculations about the nature of being and the source of the universe. Discussions of the essence of reality are found in the Upanishads, ancient Indian scriptures dating from as early as 700 BCE. They say that the universe has a divine foundation and discuss in what sense ultimate reality is one or many. Samkhya, the first orthodox school of Indian philosophy, formulated an atheist dualist ontology based on the Upanishads, identifying pure consciousness and matter as its two foundational principles. The later Vaisheshika school proposed a comprehensive system of categories. In ancient China, Laozi 's (6th century BCE) Taoism examines the underlying order of the universe, known as Tao, and how this order is shaped by the interaction of two basic forces, yin and yang. The philosophical movement of Xuanxue emerged in the 3rd century CE and explored the relation between being and non-being.

Starting in the 6th century BCE, Presocratic philosophers in ancient Greece aimed to provide rational explanations of the universe. They suggested that a first principle, such as water or fire, is the primal source of all things. Parmenides (c. 515–450 BCE) is sometimes considered the founder of ontology because of his explicit discussion of the concepts of being and non-being. Inspired by Presocratic philosophy, Plato (427–347 BCE) developed his theory of forms. It distinguishes between unchangeable perfect forms and matter, which has a lower degree of existence and imitates the forms. Aristotle (384–322 BCE) suggested an elaborate system of categories that introduced the concept of substance as the primary kind of being. The school of Neoplatonism arose in the 3rd century CE and proposed an ineffable source of everything, called the One, which is more basic than being itself.

The problem of universals was an influential topic in medieval ontology. Boethius (477–524 CE) suggested that universals can exist not only in matter but also in the mind. This view inspired Peter Abelard (1079–1142 CE), who proposed that universals exist only in the mind. Thomas Aquinas (1224–1274 CE) developed and refined fundamental ontological distinctions, such as the contrast between existence and essence, between substance and accidents, and between matter and form. He also discussed the transcendentals, which are the most general properties or modes of being. John Duns Scotus (1266–1308) argued that all entities, including God, exist in the same way and that each entity has a unique essence, called haecceity. William of Ockham (c. 1287–1347 CE) proposed that one can decide between competing ontological theories by assessing which one uses the smallest number of elements, a principle known as Ockham's razor.

In Arabic-Persian philosophy, Avicenna (980–1037 CE) combined ontology with theology. He identified God as a necessary being that is the source of everything else, which only has contingent existence. In 8th-century Indian philosophy, the school of Advaita Vedanta emerged. It says that only a single all-encompassing entity exists, stating that the impression of a plurality of distinct entities is an illusion. Starting in the 13th century CE, the Navya-Nyāya school built on Vaisheshika ontology with a particular focus on the problem of non-existence and negation. 9th-century China saw the emergence of Neo-Confucianism, which developed the idea that a rational principle, known as li, is the ground of being and order of the cosmos.

René Descartes (1596–1650) formulated a dualist ontology at the beginning of the modern period. It distinguishes between mind and matter as distinct substances that causally interact. Rejecting Descartes's dualism, Baruch Spinoza (1632–1677) proposed a monist ontology according to which there is only a single entity that is identical to God and nature. Gottfried Wilhelm Leibniz (1646–1716), by contrast, said that the universe is made up of many simple substances, which are synchronized but do not interact with one another. John Locke (1632–1704) proposed his substratum theory, which says that each object has a featureless substratum that supports the object's properties. Christian Wolff (1679–1754) was influential in establishing ontology as a distinct discipline, delimiting its scope from other forms of metaphysical inquiry. George Berkeley (1685–1753) developed an idealist ontology according to which material objects are ideas perceived by minds.

Immanuel Kant (1724–1804) rejected the idea that humans can have direct knowledge of independently existing things and their nature, limiting knowledge to the field of appearances. For Kant, ontology does not study external things but provides a system of pure concepts of understanding. Influenced by Kant's philosophy, Georg Wilhelm Friedrich Hegel (1770–1831) linked ontology and logic. He said that being and thought are identical and examined their foundational structures. Arthur Schopenhauer (1788–1860) rejected Hegel's philosophy and proposed that the world is an expression of a blind and irrational will. Francis Herbert Bradley (1846–1924) saw absolute spirit as the ultimate and all-encompassing reality while denying that there are any external relations.

At the beginning of the 20th century, Edmund Husserl (1859–1938) developed phenomenology and employed its method, the description of experience, to address ontological problems. This idea inspired his student Martin Heidegger (1889–1976) to clarify the meaning of being by exploring the mode of human existence. Jean-Paul Sartre responded to Heidegger's philosophy by examining the relation between being and nothingness from the perspective of human existence, freedom, and consciousness. Based on the phenomenological method, Nicolai Hartmann (1882–1950) developed a complex hierarchical ontology that divides reality into four levels: inanimate, biological, psychological, and spiritual.

Alexius Meinong (1853–1920) articulated a controversial ontological theory that includes nonexistent objects as part of being. Arguing against this theory, Bertrand Russell (1872–1970) formulated a fact ontology known as logical atomism. This idea was further refined by the early Ludwig Wittgenstein (1889–1951) and inspired D. M. Armstrong 's (1926–2014) ontology. Alfred North Whitehead (1861–1947), by contrast, developed a process ontology. Rudolf Carnap (1891–1970) questioned the objectivity of ontological theories by claiming that what exists depends on one's linguistic framework. He had a strong influence on Willard Van Orman Quine (1908–2000), who analyzed the ontological commitments of scientific theories to solve ontological problems. Quine's student David Lewis (1941–2001) formulated the position of modal realism, which says that possible worlds are as real and concrete as the actual world. Since the end of the 20th century, interest in applied ontology has risen in computer and information science with the development of conceptual frameworks for specific domains.