What is there? The question is deceptively simple. You might list chairs, people, electrons, nations, numbers, and God, then realize that each item on the list raises a further question: in what sense does it exist? A chair occupies space and can be sat upon; a nation has no particular location; the number seven cannot be pointed to or touched; and God, if God exists, is supposed to exist necessarily, in a way that chairs and nations do not. Before any of these questions can be answered — before we can even state them properly — we need a discipline that investigates the most general features of reality itself. That discipline is metaphysics.
Metaphysics has been treated with suspicion and with reverence in alternating historical periods. The logical positivists of the early twentieth century declared it meaningless: metaphysical claims, they argued, are neither empirically verifiable nor analytically true, and therefore say nothing. But analytic philosophy's own development undermined this verdict. As philosophers examined the logical structure of scientific theories, the semantics of modal claims, and the metaphysics implicit in ordinary language, metaphysical questions re-entered with renewed vigor. By the last decades of the twentieth century, debates about possible worlds, personal identity, causation, and time were among the most technically sophisticated in all of philosophy.
These debates are not confined to seminar rooms. Questions about personal identity bear on bioethics, criminal justice, and how we relate to our past and future selves. Questions about causation bear on scientific explanation and legal liability. Questions about the nature of time bear on physics. Metaphysics is where the most basic conceptual commitments of every intellectual discipline are laid bare and examined.
"The question of being is the darkest in all philosophy." — William James, Some Problems of Philosophy, 1911
| Metaphysical Question | Main Positions | Key Debate |
|---|---|---|
| What exists? (Ontology) | Materialism vs. idealism vs. dualism | Is reality physical, mental, or both? |
| What makes something persist over time? | Endurantism vs. perdurantism | Ship of Theseus; personal identity |
| Do abstract objects exist? | Platonism vs. nominalism | Numbers, properties, universals |
| Free will and determinism | Hard determinism; compatibilism; libertarianism | Can choices be free if caused? |
| Causation | Regularity theory; necessity; counterfactuals | What is the nature of cause and effect? |
| Time and space | Eternalism vs. presentism; substantivalism | Does time flow? Is space real? |
Key Definitions
Metaphysics: The branch of philosophy that investigates the fundamental nature and structure of reality, including questions about what kinds of things exist, the nature of causation, time, identity, and necessity.
Ontology: The sub-discipline of metaphysics concerned specifically with the question of what exists, and how different categories of things (substances, properties, relations, events) are related.
Modal metaphysics: The study of necessity and possibility; what must be the case, what could be the case, and what could not be otherwise.
Rigid designator: A term, introduced by Saul Kripke, that refers to the same entity in every possible world in which that entity exists.
The block universe: The four-dimensionalist view that past, present, and future events are equally real, arranged in a four-dimensional spacetime manifold with no privileged present moment.
Aristotle and the Invention of First Philosophy
Being Qua Being
Aristotle described the discipline he called "first philosophy" as the study of being qua being — the study of what it means to be, considered in its most general form, prior to any restriction to a particular domain. Every special science studies some portion of reality: physics studies natural bodies in motion, mathematics studies quantity, biology studies living things. First philosophy has no such restriction; it asks what is true of whatever exists, simply in virtue of its existing.
The title "metaphysics" was not Aristotle's own coinage. It derives from the arrangement of his manuscripts by the editor Andronicus of Rhodes in the first century BCE, who placed the books on first philosophy after (meta) the books on physics (ta physika). The word "metaphysics" thus began as a cataloguing accident and became the name of an inquiry.
Aristotle's first philosophy addresses the categories of being — substance, quantity, quality, relation, place, time, position, state, action, passion — with substance (ousia) as primary. Individual substances, such as this horse or this human being, are the fundamental units of reality; other categories exist only as features of substances. The study of substance, its essential nature, and the principles of change is at the heart of Aristotelian metaphysics.
Kant's Critical Limits
Immanuel Kant, in the Critique of Pure Reason (1781), argued that the ambitions of rationalist metaphysics — to know the nature of God, the soul, and the world as a whole through pure reason — are systematically mistaken. The categories of the understanding (causation, substance, unity) are applicable only within the bounds of possible experience; they are forms that the mind imposes on sensory data to make experience coherent. When these categories are applied beyond experience — to things-in-themselves, to the universe as a whole, or to God — the result is not knowledge but antinomies: pairs of contradictory positions, each apparently supported by valid arguments. Kant did not abandon metaphysics but deflated its pretensions: a critique of the limits of reason is itself a metaphysical achievement.
Ontology: What Exists?
Quine's Framework
W.V.O. Quine posed the question of ontology with deliberate starkness in "On What There Is" (1948): to be is to be the value of a variable. A theory is committed to the existence of whatever entities its quantified statements must range over if the theory is true. Quine's framework allows us to read off ontological commitments from theoretical discourse: if our best physics quantifies over spacetime points, we are committed to them; if our best mathematics quantifies over sets, we are committed to sets.
Quine himself was a physicalist and naturalist: he accepted the ontology of physics and resisted the inflation of our ontological inventory beyond what natural science requires. His criterion of ontological parsimony — do not multiply entities beyond necessity — aligns with the scientific virtue of theoretical economy.
The Abstract Objects Debate
Do abstract objects — numbers, sets, propositions, properties — exist? And if they exist, in what sense? Platonism about abstract objects holds that they exist necessarily, independently of minds and language, in a non-spatial, non-temporal realm. Mathematical truths on this view are discovered, not invented: the fact that there are infinitely many prime numbers was true before any mathematician formulated it.
Nominalism denies abstract objects. In various forms, it holds that only particular concrete objects exist, and that the apparent reference to abstract objects in mathematics and logic can be paraphrased away, construed as fiction, or explained by appeal to language and concepts rather than abstract entities.
Hartry Field's fictionalism, developed in Science Without Numbers (1980), represents a sophisticated nominalist position: mathematical statements are not literally true, but mathematics is useful as a fiction that facilitates calculation over and above what a purely nominalistic physics could express.
David Armstrong defended a moderate realism about universals in Universals and Scientific Realism (1978). Universals — properties and relations — exist, but only as constituents of states of affairs: the redness of this apple is the universal redness instantiated in this particular. There are no uninstantiated universals, unlike Platonic forms that exist independently of their instances.
Personal Identity
Locke, Hume, and the Self
John Locke, in An Essay Concerning Human Understanding (1689), proposed that personal identity consists in continuity of consciousness, specifically memory. A person at a later time is the same person as a person at an earlier time if and only if the later person remembers the experiences of the earlier person. This memory criterion accounted for our ordinary practices: we hold people responsible for past actions because they remember doing them; amnesia complicates responsibility because the memory link is broken.
Joseph Butler and Thomas Reid objected that Locke's account is circular: memory already presupposes personal identity rather than constituting it. To remember doing something is to remember that I did it, which means that I am already identified as the person who had that experience. Memory cannot be the criterion of personal identity if the concept of memory already invokes personal identity.
David Hume, in the Treatise of Human Nature (1739), took a more radical path. Introspection, he argued, never reveals a persisting self — only a stream of particular perceptions. The self is a fiction constructed by the mind's tendency to associate similar and causally related perceptions into a bundle. There is no Cartesian ego over and above the bundle of experiences.
Parfit's Revolution
Derek Parfit's Reasons and Persons (1984) is among the most provocative works of twentieth-century philosophy. Parfit argued that our ordinary concept of personal identity is both indeterminate in hard cases and not what matters for the things we care about in survival.
His central thought experiments exploit the distinction between numerical identity (the strict relation of being one and the same thing) and psychological continuity (the overlapping chain of memories, intentions, beliefs, and character traits that connect person-stages). In the fission case: a person's brain is divided and each hemisphere transplanted into a different body. Both resulting persons have equal psychological continuity with the original. If identity requires uniqueness (no person can be identical to two distinct people), then neither resulting person is strictly identical to the original — yet each has as much of what survival normally provides as ordinary survival does.
Parfit's reductionist conclusion: persons are not fundamental ontological units. They are just series of physical and mental events with no further fact distinguishing them. Personal identity is not a deep metaphysical relation but a conventional matter that admits of indeterminate cases. What matters is not identity but the degree of psychological continuity and connectedness.
This conclusion has sweeping implications for ethics. If there is no deep personal identity across time, then purely self-interested reasons (why should I care especially about my future self?) are undermined. And if persons are not fundamental, then utilitarian aggregation across persons is less problematic than it appears. Parfit pursued these implications in the later chapters of Reasons and Persons, arguing toward a convergence of self-interest and consequentialism.
Causation and Necessity
Hume's Challenge
Hume's analysis of causation in the Enquiry Concerning Human Understanding (1748) identified what has become one of the central problems of metaphysics. We believe that causes necessitate their effects — that given the cause, the effect must follow. But when we examine our experience, Hume argues, we find only constant conjunction and temporal priority: B regularly follows A, and A precedes B. We never observe the necessitation itself. Our idea of necessary connection is a projection of psychological expectation — the habit of the mind — onto the world.
Hume's analysis leaves causation without objective modal force. On a Humean regularity theory, causal laws are just regularities: "all Fs are Gs" without any deeper necessitation. This has seemed unsatisfying to many philosophers who hold that causal laws do more than merely describe what happens to occur.
Counterfactual and Dispositional Accounts
David Lewis developed a counterfactual analysis of causation: C causes E if and only if, had C not occurred, E would not have occurred. Lewis's possible-worlds semantics provided the apparatus for evaluating such counterfactuals: a counterfactual is true if the closest possible worlds where the antecedent is true are worlds where the consequent is also true. This elegant analysis captures much of our causal reasoning but faces difficulties with preemption cases (where a backup cause stands ready) and overdetermination (where two simultaneous causes each suffice for the effect).
Dispositionalism, associated with Sydney Shoemaker, C.B. Martin, and Alexander Bird, holds that causal powers and dispositions are fundamental features of properties. Fragility, for instance, is a real disposition of glass; it constitutes the glass's power to break under impact. On this view, causal laws are grounded in the essential natures of properties, giving modal force to causal relations without invoking Humean regularity or Newtonian forces as unexplained primitives.
Possible Worlds and Kripkean Necessity
Saul Kripke's Naming and Necessity (1980, based on lectures delivered in 1970) transformed both philosophy of language and metaphysics. Kripke distinguished the metaphysical distinction between necessary and contingent truths from the epistemological distinction between a priori and a posteriori knowledge, arguing that these two distinctions do not coincide.
Rigid designators — terms that refer to the same entity in every possible world — include proper names and natural kind terms. Because "water" rigidly designates the actual substance whose molecular structure is H2O, "water is H2O" is necessarily true: there is no possible world in which the very substance we call water fails to be H2O. Yet this is discovered empirically — we could not have known it by conceptual analysis. Necessary truths can be known only a posteriori.
This result rehabilitated essentialism: things have essential properties, properties they could not lack while remaining the things they are. Aristotle is essentially human; the gold in this ring is essentially gold (it could not have been made of iron and been the same ring in some possible world). Kripke's metaphysics opened a research program in the study of essence that continues to be productive.
The Free Will Debate
Determinism and Its Discontents
The free will debate is structured by a conditional: if determinism is true (every event is causally necessitated by prior events and laws of nature), then no one could have acted otherwise, and therefore no one is genuinely responsible for anything. This argument, stated most rigorously by Peter van Inwagen in An Essay on Free Will (1983) as the Consequence Argument, concludes that free will and determinism are incompatible.
Hard determinists accept incompatibilism and determinism, concluding that free will is an illusion. This view is consistent but has implications most find intolerable: punishment, praise, gratitude, and resentment lose their rationale if no one could have done otherwise.
Compatibilism
Compatibilism holds that the relevant kind of free will — the kind required for moral responsibility — is compatible with determinism. A free action is not one that escapes the causal order but one caused in the right way: by the agent's own deliberation, values, and character, without external compulsion or internal dysfunction such as phobia or compulsion.
P.F. Strawson's "Freedom and Resentment" (1962) offered a compelling defense that bypassed the traditional debate. Our practices of holding responsible — praising and blaming, feeling gratitude and resentment and indignation — are not contingent policies adopted on the basis of metaphysical theory. They are constitutive of our interpersonal lives and our very identity as moral beings. To ask whether determinism undermines these practices is to ask whether a theoretical discovery about the causal structure of events could rationally lead us to abandon the human form of life. Strawson thought this impossible.
Harry Frankfurt's hierarchical theory locates free will in the alignment of an agent's will with their reflectively endorsed desires. A person acts freely when the desire that moves them is one they endorse as their own at the second-order level — when they want to want to act as they do. An addict who is moved by a craving they desperately wish they did not have lacks free will; a person who acts from desires they endorse on reflection acts freely, whether or not determinism is true.
Frankfurt cases challenge the principle of alternative possibilities: a neuroscientist monitoring an agent's brain states is prepared to intervene and cause the agent to choose A if they are about to choose B — but the agent chooses A anyway, without intervention. The agent could not have done otherwise (the counterfactual intervener would have ensured A regardless) yet seems responsible for choosing A.
Time and Space
McTaggart's Analysis
J.M.E. McTaggart's "The Unreality of Time" (1908) introduced the distinction between the A-series and the B-series that has structured the metaphysics of time ever since. The A-series orders events as past, present, and future — positions that change as time passes (what is now future becomes present and then past). The B-series orders events as earlier than, simultaneous with, and later than other events — relations that are permanent (the battle is always earlier than the armistice).
McTaggart argued that the A-series is essential to time because time without genuine temporal passage — without the distinction between a living present and a dead past — would be indistinguishable from spatial order. Yet the A-series generates contradiction: every event must be simultaneously past, present, and future, which are incompatible properties. The standard response — that each event has these properties at different times — generates a regress. McTaggart's conclusion was that time is unreal. Few have accepted this conclusion, but his distinction remains indispensable.
Presentism holds that only present entities and events exist; past and future are unreal. Eternalism (the block universe) holds that all temporal positions are equally real; the present is no more privileged than any spatial position. Special relativity supports eternalism: there is no absolute simultaneity, no frame-independent present moment. The growing block theory offers a middle position: past and present are real, future is not.
Absolute vs. Relational Space
The debate between Newton and Leibniz about the nature of space remains relevant. Newton, in the Principia Mathematica (1687), argued for absolute space: a real, infinite, uniform container that would exist even if it contained no matter. Absolute space grounds the distinction between genuine acceleration and merely relative motion. Leibniz objected, in correspondence with Samuel Clarke, on the principle of sufficient reason: if space were absolute, God would have had no reason to place the universe here rather than there, since both positions would be indistinguishable. Space is relational: it consists only in the spatial relations among existing bodies.
Ernst Mach in the nineteenth century renewed the relational view and influenced Einstein, whose general theory of relativity replaced absolute spacetime with dynamical, curved spacetime shaped by the distribution of mass and energy.
Philosophy of Religion
Arguments for God's Existence
The three main arguments for the existence of God have been subjected to centuries of analysis. The ontological argument, formulated by Anselm of Canterbury in the Proslogion (1078), argues from the concept of God to God's existence: God is defined as the greatest conceivable being; a being that exists in reality is greater than one that exists only in thought; therefore God must exist in reality. Kant's objection — that existence is not a predicate, adds nothing to a concept, and therefore cannot be part of the essence of any being — was widely accepted. Alvin Plantinga's modal ontological argument (The Nature of Necessity, 1974) reconstructs the argument using possible-worlds semantics: if maximal greatness is possible (if there is some possible world where a maximally great being exists), then by the logic of modal realism, a maximally great being exists in all possible worlds, including the actual one. The argument is valid; the dispute focuses on whether maximal greatness is genuinely possible.
The cosmological argument appeals to the existence of the world as evidence for a first cause. Aquinas's Five Ways in the Summa Theologiae (c. 1265-1274) include arguments from motion, efficient causation, and contingency. The Kalam cosmological argument, given contemporary defense by William Lane Craig in The Kalam Cosmological Argument (1979), runs: whatever begins to exist has a cause; the universe began to exist; therefore the universe has a cause external to itself. Critics dispute the second premise (quantum cosmology suggests possibilities for uncaused beginnings) and question the inference from "first cause" to the God of theism.
The Problem of Evil
J.L. Mackie's "Evil and Omnipotence" (1955) presented the logical problem of evil as a formal inconsistency: the propositions "God is omnipotent," "God is omniscient," "God is omnibenevolent," and "Evil exists" cannot all be true simultaneously. An omnipotent and omnibenevolent God would prevent all evil; therefore an omniscient God knowing of evil's existence would eliminate it. The existence of evil is therefore incompatible with such a God.
Plantinga's free will defense, developed in God, Freedom, and Evil (1974), argues that God could not have created free beings who always choose rightly. Transworld depravity — the claim that every possible free creature goes wrong in at least some possible world — is not a contradiction, and if it holds, then no possible world with free creatures is free of all evil. The logical problem of evil, Plantinga argued, is not decisive because the free will defense provides a possible reason God might have for permitting evil.
John Hick's soul-making theodicy, developed in Evil and the God of Love (1966), argues that a world without evil could not serve the purposes of moral and spiritual development. Virtues like courage, compassion, and perseverance require adversity. A painless world of automatic goodness would preclude the kind of character formation that Hick takes to be central to the human vocation.
References
- Aristotle. Metaphysics. Trans. W.D. Ross. In The Complete Works of Aristotle, ed. Jonathan Barnes. Princeton University Press, 1984.
- Kant, Immanuel. Critique of Pure Reason. Trans. Paul Guyer and Allen Wood. Cambridge University Press, 1998. (Original 1781.)
- Quine, W.V.O. "On What There Is." Review of Metaphysics, vol. 2, no. 5, 1948, pp. 21-38.
- Armstrong, David. Universals and Scientific Realism. 2 vols. Cambridge University Press, 1978.
- Parfit, Derek. Reasons and Persons. Oxford University Press, 1984.
- van Inwagen, Peter. An Essay on Free Will. Oxford University Press, 1983.
- Strawson, P.F. "Freedom and Resentment." Proceedings of the British Academy, vol. 48, 1962, pp. 1-25.
- Frankfurt, Harry. "Freedom of the Will and the Concept of a Person." Journal of Philosophy, vol. 68, no. 1, 1971, pp. 5-20.
- McTaggart, J.M.E. "The Unreality of Time." Mind, vol. 17, no. 68, 1908, pp. 457-474.
- Kripke, Saul. Naming and Necessity. Harvard University Press, 1980. (Original lectures 1970.)
- Mackie, J.L. "Evil and Omnipotence." Mind, vol. 64, no. 254, 1955, pp. 200-212.
- Plantinga, Alvin. The Nature of Necessity. Oxford University Press, 1974.
- Hick, John. Evil and the God of Love. Macmillan, 1966.