Necessity and Possibility

To say something is necessary is to say it could not have been otherwise; to say it is possible is to say it could obtain. This topic distinguishes kinds of necessity and examines how we know modal truths.

Definition

Necessity is truth that could not fail to hold; possibility is consistency with how things could be. Different kinds of necessity correspond to different ranges of alternatives held fixed.

Scope

Covers logical, metaphysical, and physical necessity, the de re and de dicto distinction, the relations among necessity, the a priori and the analytic, and the epistemology of modality including conceivability arguments.

Core questions

• What kinds of necessity are there, and how do they relate?
• Does necessity coincide with the a priori or the analytic?
• Can there be necessary truths known only a posteriori?
• How do we come to know what is possible or necessary?

Key concepts

• Metaphysical necessity
• Logical necessity
• Physical necessity
• De re and de dicto
• Conceivability
• A priori and a posteriori

Key theories

A posteriori necessity

Kripke argued that some necessary truths, such as identity statements and claims about origins or natural kinds, can be discovered only through experience, breaking the traditional tie between necessity and apriority.

Varieties of necessity

Fine distinguishes metaphysical, natural, and normative necessity as irreducibly different sources of necessity rather than restrictions of a single modal notion.

History

Kant tied necessity to the a priori and the analytic. The twentieth century separated these notions: Quine attacked analyticity, while Kripke argued for necessary a posteriori and contingent a priori truths. Contemporary work distinguishes several kinds of necessity and develops the epistemology of modality.

Debates

Does conceivability entail possibility?

Some philosophers treat what is coherently conceivable as a guide to what is metaphysically possible; critics argue conceivability is fallible and that a posteriori necessities show conceivable scenarios can be impossible.

Key figures

• Immanuel Kant
• Saul Kripke
• Kit Fine
• David Chalmers
• Ruth Barcan Marcus

Seminal works

• kripke1980
• fine2002

Frequently asked questions

Is everything necessary either known a priori?

No. Following Kripke, many philosophers accept necessary a posteriori truths, such as that water is H2O, which are necessary yet knowable only through empirical investigation.