Modality and Possible Worlds
Modal metaphysics concerns necessity and possibility: what must be the case, what could have been otherwise, and how to analyze such claims. Possible worlds are the standard tool for representing modal facts.
Definition
Modality is the study of the modes of truth, principally necessity and possibility, and of the ontology and semantics used to represent them, such as possible worlds.
Scope
Covers the interpretation of necessity and possibility, possible-worlds semantics for modal logic, de re modality and essence, and the metaphysical status of possible worlds in modal realism and actualism.
Sub-topics
Core questions
- What is it for something to be necessary or possible?
- Are possible worlds real entities, or useful fictions?
- Do things have essential properties they could not have lacked?
- How should de re modal claims about individuals be understood?
Key concepts
- Necessity
- Possibility
- Possible world
- Rigid designation
- De re and de dicto
- Essence
- Counterpart
Key theories
- Modal realism
- Lewis holds that possible worlds are concrete realities as real as the actual world; the actual world is just the one we inhabit, and modal claims are quantifications over these worlds and their inhabitants.
- Rigid designation and a posteriori necessity
- Kripke argues that names are rigid designators picking the same individual in every world, yielding necessary truths knowable only a posteriori, such as identities of kinds and origins.
- Essence as prior to modality
- Fine argues that essence cannot be reduced to necessity: an object's essence concerns what it is to be that object, and grounds rather than coincides with the necessary truths about it.
History
Leibniz introduced talk of possible worlds. The development of modal logic in the mid-twentieth century by Carnap, Barcan Marcus, and Kripke provided a formal semantics in terms of possible worlds. Kripke's Naming and Necessity reshaped modal metaphysics, Lewis defended full-blooded modal realism, and Fine and others revived essence as a notion distinct from necessity.
Debates
- The reality of possible worlds
- Modal realists treat possible worlds as concrete existing entities; actualists hold that only the actual world is concrete and reconstruct worlds as abstract objects such as maximal states of affairs.
Key figures
- Gottfried Wilhelm Leibniz
- Saul Kripke
- David Lewis
- Alvin Plantinga
- Kit Fine
- Ruth Barcan Marcus
Related topics
Seminal works
- lewis1986
- kripke1980
- plantinga1974
Frequently asked questions
- What is a possible world?
- A possible world is a complete way things could have been. Philosophers use possible worlds to analyze modal claims: a proposition is necessary if true in all worlds and possible if true in some. They disagree over whether such worlds are concrete or abstract.