Key Figures

A compact reference to the principal contributors, with the claim each is cited for in this section. Arranged chronologically. The narrative thread is on the history page.

Ancient and medieval

• Parmenides (early 5th c. BCE) — the argument that what is not cannot be thought, hence that change and plurality are impossible; the origin of the problem of negative existentials and of Plato's beard.
• Plato (c. 428–348 BCE) — the Forms as the answer to the one over many; transcendent realism; the third man objection in the Parmenides.
• Aristotle (384–322 BCE) — the ten categories; primary substance; hylomorphism; "being is said in many ways"; immanent realism about universals.
• Porphyry (c. 234–305) — the three questions about genera and species that framed the medieval problem of universals.
• Boethius (c. 477–524) — transmitted Porphyry's questions to the Latin West.
• Avicenna / Ibn Sīnā (c. 980–1037) — the essence/existence distinction; essence neutral as to existence and universality.
• Abelard (1079–1142) — early non-realism about universals: what is shared is a status, not a thing.
• Aquinas (1225–1274) — the real distinction between essence and esse in creatures; their identity in God.
• Duns Scotus (c. 1266–1308) — haecceity as the principle of individuation; the univocity of being against the analogists.
• Ockham (c. 1287–1347) — rigorous nominalism; the parsimony principle.

Early modern

• Descartes (1596–1650) — two created substances distinguished by principal attributes; the mind–body problem as a consequence of the scheme.
• Spinoza (1632–1677) — substance monism from the independence criterion taken literally.
• Locke (1632–1704) — real versus nominal essence; the substratum as "something, we know not what."
• Leibniz (1646–1716) — monads as true unities; the Identity of Indiscernibles; possible worlds as ideas in the divine mind.
• Hume (1711–1776) — the bundle theory of objects and of the self; no impression of substance or of necessary connection.
• Kant (1724–1804) — categories as conditions of experience rather than divisions of being; existence is not a real predicate.

The two traditions

• Brentano (1838–1917) — intentionality: every mental act is directed at an object, which makes non-existent objects a problem.
• Frege (1848–1925) — existence as a second-order property expressed by the quantifier; abstraction principles; the ancestor of neo-Fregeanism.
• Meinong (1853–1920) — the theory of objects: independence of Sosein from Sein; Aussersein.
• Husserl (1859–1938) — formal versus regional ontology.
• Russell (1872–1970) — the theory of descriptions; logical atomism and facts; the objections to Meinong.
• Leśniewski (1886–1939) — mereology as a nominalist alternative to set theory.
• Wittgenstein (1889–1951) — "the world is the totality of facts, not of things."
• Carnap (1891–1970) — linguistic frameworks and the internal/external distinction; the 1932 attack on Heidegger.
• Heidegger (1889–1976) — the ontological difference and the Seinsfrage; ready-to-hand versus present-at-hand.

The Quinean era

• Quine (1908–2000) — to be is to be the value of a bound variable; the attack on the analytic/synthetic distinction; ontological relativity and proxy functions.
• Goodman (1906–1998) — the calculus of individuals; the companionship and imperfect-community problems for resemblance nominalism.
• Strawson (1919–2006) — descriptive versus revisionary metaphysics.
• D. C. Williams (1899–1983) — tropes as "the alphabet of being."
• Davidson (1917–2003) — events from adverbial modification; the slingshot.
• Putnam (1926–2016) — Twin Earth and natural-kind externalism; the model-theoretic argument; conceptual relativity.
• Kripke (1940–2022) — rigid designation; the necessity of identity; necessary a posteriori and contingent a priori; origin and kind essentialism.
• Armstrong (1926–2014) — a posteriori realism about universals; states of affairs; the truthmaker principle; laws as relations among universals; the "ostrich" charge.
• Lewis (1941–2001) — modal realism and counterpart theory; naturalness; the ontological innocence of mereology; the best-system account of laws; Humean supervenience; "Holes."
• Field (b. 1946) — nominalist physics and conservativeness; the reliability challenge to platonism.
• van Inwagen (b. 1942) — the Special Composition Question; meta-ontology as a Quinean package.
• van Fraassen (b. 1941) — constructive empiricism; the identification and inference problems for laws.
• Laudan (b. 1941) — the pessimistic meta-induction.
• Hacking (1936–2023) — entity realism; the taxonomy of social construction and interactive kinds.
• Cartwright (b. 1944) — the dappled world; laws as ceteris paribus; entity realism.
• Searle (b. 1932) — constitutive rules and institutional facts.
• Parfit (1942–2017) — fission and the claim that identity is not what matters.

Contemporary

• Fine (b. 1945) — essence prior to modality; grounding as an operator; the critique of the modal analysis of dependence.
• Sider (b. 1967) — structure as primitive; the quantifier as joint-carving; the argument from vagueness.
• Schaffer (b. 1973) — priority monism; "Is there a fundamental level?"; grounding as a relation.
• Thomasson (b. 1968) — easy ontology; fictional characters as abstract artifacts.
• Hirsch (b. 1939) — quantifier variance.
• Wright and Hale — neo-Fregeanism; Hume's Principle and Frege's Theorem.
• Zalta (b. 1952) — encoding versus exemplifying; abstract objects theory.
• Priest (b. 1948) — noneism and paraconsistent treatment of impossible objects.
• Rodriguez-Pereyra (b. 1968) — resemblance nominalism defended.
• Lowe (1950–2014) — the four-category ontology; substance as irreducible.
• Saunders (b. 1954) — weak discernibility.
• French and Ladyman — ontic structural realism; quasi-set theory; naturalised metaphysics.
• Korman — conservatism about ordinary objects and the reply to debunking arguments.
• McDaniel and Turner — ontological pluralism, and its Heideggerian source.
• Wallace (b. 1976) — Everettian real patterns and functionalism about macroscopic objects.
• Wilson and Hofweber — scepticism about grounding and about esoteric metaphysics.