A proposition is a metaphysical necessity if it could not have been false. But there are various 'strengths' of necessity. In some sense, it necessarily takes longer than a day to get to the moon, because we don't have fast enough rockets to get us there any quicker. But in another sense, we could get to the moon quicker - if we had quicker rockets. But even with faster rockets, it necessarily takes longer than a second - for necessarily, the fastest we could travel is at the speed of light. But again: that's only necessary given the laws of nature. There's certainly no logical contradiction in travelling to the moon in a nano-second. In this sense of necessity, what's necessary are claims like 'if I travel to the moon, then I travel to the moon' - claims whose truth follows from logic alone.
Call the three strengths of necessity above 'practical', 'nomological' and 'logical' necessity respectively. Each of them is a 'relative' necessity in the sense that they don't say what is necessary simpliciter: they say what is necessary given certain other facts (facts concerning what's practically available to us, the laws of nature, the laws of logic, respectively). 'Metaphysical' necessity, by contrast, is meant to be necessity simpliciter: what's metaphysically necessary isn't just what's necessary given some other facts, but what's necessary simpliciter.
This is not to say that if something is metaphysically necessary, it is ipso facto necessary in all other senses as well. To the contrary, something can be metaphysically necessary yet logically non-necessary (i.e. logically contingent). For example, it's common to adduce the statement "God exists" as an instance of something which is metaphysically necessary but not logically necessary.
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