In philosophy, metaphysical necessity, sometimes called broad logical necessity,is one of many different kinds of necessity, which sits between logical necessity and nomological (or physical) necessity, in the sense that logical necessity entails metaphysical necessity, but not vice versa, and metaphysical necessity entails physical necessity, but not vice versa. A proposition is said to be necessary if it could not have failed to be the case. Nomological necessity is necessity according to the laws of physics and logical necessity is necessity according to the laws of logic, while metaphysical necessities are necessary in the sense that the world could not possibly have been otherwise. What facts are metaphysically necessary, and on what basis we might view certain facts as metaphysically but not logically necessary are subjects of substantial discussion in contemporary philosophy.
The concept of a metaphysically necessary being plays an important role in certain arguments for the existence of God, especially the ontological argument, but metaphysical necessity is also one of the central concepts in late 20th century analytic philosophy. Metaphysical necessity has proved a controversial concept, and criticized by David Hume, Immanuel Kant, J. L. Mackie, and Richard Swinburne, among others.
Metaphysical necessity is contrasted with other types of necessity. For example, the philosophers of religion John Hickand William L. Rowe distinguished the following three:
While many theologians (e.g. Anselm of Canterbury, René Descartes, and Gottfried Leibniz) considered God to be a logically or metaphysically necessary being, Richard Swinburne argued for factual necessity, and Alvin Plantinga argues that God is a causally necessary being. Because a factually or causally necessary being does not exist by logical necessity, it does not exist in all logically possible worlds.Therefore, Swinburne used the term "ultimate brute fact" for the existence of God.
In Naming and Necessity,Saul Kripke argued that there were a posteriori truths, such as Hesperus is Phosphoros, or Water is H₂O, that were nonetheless metaphysically necessary.