Metaphysics

The central question of metaphysics is the ontological question: what exists? W.V.O. Quine gave a famous answer to "what is there?" — "Everything" — and the laconic follow-up: the interesting question is what kinds of things are included in "everything." Do numbers exist? Do properties exist? Do possible worlds exist? Do fictional characters exist? Each answer generates a different ontology with different philosophical consequences.

Physicalism (or materialism) holds that only physical things exist: matter, energy, space, time, and whatever science ultimately tells us these are. Everything else — minds, properties, numbers — either reduces to physical things or is eliminated. This is now the dominant view in analytic philosophy and is continuous with scientific practice.

Idealism holds that mental entities are the fundamental constituents of reality. In its strong form (Berkeley), nothing exists except minds and their contents. In its weaker form (Kant), we can only know mind-structured experience, not the thing-in-itself. German Idealism (Hegel, Fichte, Schelling) developed elaborate systems in which Absolute Spirit unfolds through history.

Dualism holds that there are two fundamental kinds of substance: mental and physical. Descartes' version posits soul and body as radically different kinds of thing. Property dualism (contemporary version) holds there is only one kind of substance (physical) but two irreducible kinds of property: physical and phenomenal.

Metaphysics also examines the structure of time. Does the past still exist? Does the future yet exist? Presentism holds that only the present moment exists. Eternalism (or the "block universe" view) holds that past, present, and future all equally exist — temporal position is like spatial position, a matter of where in the block you are. Special relativity seems to favor eternalism, since "simultaneous" is relative to reference frame and there is no privileged "now."

Causation is another central problem. Hume showed that we never observe necessary connection — only constant conjunction. Contemporary approaches include counterfactual analysis (X caused Y if, had X not occurred, Y would not have occurred), mechanistic accounts, and probabilistic accounts. The problem of backward causation, overdetermination, and prevention make causation surprisingly difficult to analyze.

Modal metaphysics asks about necessity and possibility. What does it mean to say something is necessarily true (true in all possible worlds) versus contingently true (true in this world but not others)? David Lewis's modal realism — the bold claim that all possible worlds are equally real, concrete universes — gave modal logic a vivid metaphysical backing while striking many as obviously too ontologically expensive.

What makes you the same person you were ten years ago? This is the puzzle of personal identity — a question at the intersection of metaphysics and ethics (if you are not the same person who committed a crime, should you be punished for it?). John Locke proposed that personal identity consists in psychological continuity — specifically, memory: you are the same person as your past self if and only if you can remember her experiences.

Hume's bundle theory dissolves the self entirely: there is no persistent self, only a bundle of connected perceptions. Derek Parfit's extraordinary book Reasons and Persons (1984) argued that personal identity is not what matters — what matters is psychological continuity and connectedness, and these come in degrees. If psychological continuity gradually fades (as in advanced dementia), there is no sharp fact about whether you are the same person. And if identity is not what matters, many of our intuitions about self-interest, survival, and desert need revision.