A proposition is a metaphysical necessity if it could not have been false. The concept of a metaphysically necessary being plays an important role in the ontological argument for the existence of God. This concept has been criticized and partly rejected as incoherent by David Hume, Immanuel Kant, J. L. Mackie and Richard Swinburne. The philosophers of religion John Hick and William L. Rowe distinguished three different types of necessary existence:
- factual necessity (existential necessity): a factually necessary being is not causally dependent on any other being, while any other being is causally dependent on it.
- causal necessity (subsumed by Hicks under the former type): a causally necessary being is such that it is logically impossible for it to be causally dependent on any other being, and it is logically impossible for any other being to be causally independent of it.
- logical necessity: a logically necessary being is a being whose non-existence is a logical impossibility, and which therefore exists either timeless or eternally in all possible worlds.
While most theologians (e.g. Anselm of Canterbury, René Descartes, and Gottfried Leibniz) considered God as logically 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 possible worlds. Therefore, Swinburne used the term "ultimate brute fact" for the existence of God.
- John Hick (1961): Necessary Being. - Scottish Journal of Theology, 1961: 353-369.
- William L. Rowe (1998): The Cosmological Argument. Fordham Univ Press, 273 pp.
- Ronald H. Nash (1983): The Concept of God. Grand Rapids, MI: Zondervan, p. 108
- Richard Swinburne (2004): The Existence of God. 2nd ed. Oxford: Oxford University Press, p. 96