The Possible & the Necessary

Eight steps into modality: what could have been, what must be, and whether other worlds are real.

A course on the thought of Metaphysics.

What you will learn

  • Distinguish necessity, contingency, and possibility, with ordinary examples.
  • See the danger of fatalism: if 'there will be a sea-battle' is already true, must the battle come?
  • Grasp 'necessary when it is' vs 'necessary simply' — the first tool against collapsing all into necessity.
  • State the two kinds of truth and the two great principles (contradiction; sufficient reason).
  • See the threat: if every truth has a sufficient reason, is every truth necessary (Spinoza's necessitarianism)?
  • Grasp Leibniz's escape: contingent truths are contained in the subject but recoverable only by an infinite analysis God alone completes.
  • Understand a possible world as a complete, compossible alternative arrangement of things in God's understanding.
  • State the principle of the best: God actualizes the best among all possible worlds.

Course outline

  1. Could It Have Been Otherwise? — The modal intuition and its first dangers. Some things must be, some could have been otherwise — but is the future already fixed? Aristotle's sea-battle opens the problem of necessity, contingency…
  2. Truths of Reason, Truths of Fact — Leibniz's foundational division: necessary truths of reasoning (whose opposite is a contradiction) vs contingent truths of fact (grounded in the principle of sufficient reason). The threat that the…
  3. Possible Worlds & the Best of All — Leibniz's great picture: in God's understanding lie infinitely many possible worlds, complete and compossible alternatives; by the principle of the best, God actualizes the one with the most…
  4. Is Necessity in the World, or in Us? — Kant relocates necessity. The analytic/synthetic and a priori/a posteriori distinctions; necessity and strict universality as the marks of a priori knowledge; modality…
  5. Naming and Necessity — Kripke's revolution. Names are rigid designators — they pick out the same thing in every possible world — and so identity statements between names (Hesperus = Phosphorus) and theoretical identities…
  6. Are Possible Worlds Real Places? — Lewis's modal realism: the other possible worlds are not abstractions or ways God imagines things — they are real, concrete universes, as real as ours, just spatiotemporally and causally isolated…
  7. Worlds Without the Worlds — The respectable alternative to Lewis: actualism. Possible worlds are abstract objects — maximal possible states of affairs — and only the actual world is concrete. Transworld identity and essences.…

Works read in this course

  • Aristotle, De Interpretatione 9 — future contingents, the sea-battle; necessity 'when it is'
  • Leibniz, Monadology / Theodicy — truths of reason vs fact; PSR; possible worlds; the best
  • Kant, Critique of Pure Reason — analytic/synthetic; a priori/a posteriori; the modal postulates
  • Kripke, Naming and Necessity (1980) — rigid designation; the necessary a posteriori; essentialism
  • D. Lewis, On the Plurality of Worlds (1986) — modal realism; the incredulous stare; the case from utility
  • Plantinga, The Nature of Necessity (1974) — worlds as states of affairs; transworld identity; the modal ontological argument

Terms introduced

necessity, contingency, possibility, de dicto / de re, possible world, principle of sufficient reason, truths of reason / truths of fact, analytic / synthetic, a priori / a posteriori, rigid designator, necessary a posteriori, modal realism, actualism / ersatzism, essence (de re), transworld identity

Part of the Simposeum course library.