Metaphysical necessity
Encyclopedia
A proposition is necessary 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 [i]ipso facto[/i] 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
Ontological argument
The ontological argument for the existence of God is an a priori argument for the existence of God. The ontological argument was first proposed by the eleventh-century monk Anselm of Canterbury, who defined God as the greatest possible being we can conceive...

 for the existence of God. This concept has been criticized and partly rejected as incoherent by David Hume
David Hume
David Hume was a Scottish philosopher, historian, economist, and essayist, known especially for his philosophical empiricism and skepticism. He was one of the most important figures in the history of Western philosophy and the Scottish Enlightenment...

, Immanuel Kant
Immanuel Kant
Immanuel Kant was a German philosopher from Königsberg , researching, lecturing and writing on philosophy and anthropology at the end of the 18th Century Enlightenment....

, J. L. Mackie
J. L. Mackie
John Leslie Mackie was an Australian philosopher, originally from Sydney. He made significant contributions to the philosophy of religion, metaphysics, and the philosophy of language, and is perhaps best known for his views on meta-ethics, especially his defence of moral skepticism.He authored six...

 and Richard Swinburne
Richard Swinburne
Richard G. Swinburne is an Emeritus Professor of Philosophy at the University of Oxford. Over the last 50 years Swinburne has been a very influential proponent of philosophical arguments for the existence of God. His philosophical contributions are primarily in philosophy of religion and...

. The philosophers of religion John Hick
John Hick
Professor John Harwood Hick is a philosopher of religion and theologian. In philosophical theology, he has made contributions in the areas of theodicy, eschatology, and Christology, and in the philosophy of religion he has contributed to the areas of epistemology of religion and religious...

 and William L. Rowe
William L. Rowe
William Leonard Rowe is a professor emeritus of philosophy at Purdue University who specialises in the philosophy of religion. His work has played a leading role in the "remarkable revival of analytic philosophy of religion since the 1970s"...

 distinguished three different types of necessary existence:
  1. 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.
  2. 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.
  3. logical necessity
    Logical truth
    Logical truth is one of the most fundamental concepts in logic, and there are different theories on its nature. A logical truth is a statement which is true and remains true under all reinterpretations of its components other than its logical constants. It is a type of analytic statement.Logical...

    : 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
Anselm of Canterbury
Anselm of Canterbury , also called of Aosta for his birthplace, and of Bec for his home monastery, was a Benedictine monk, a philosopher, and a prelate of the church who held the office of Archbishop of Canterbury from 1093 to 1109...

, René Descartes
René Descartes
René Descartes ; was a French philosopher and writer who spent most of his adult life in the Dutch Republic. He has been dubbed the 'Father of Modern Philosophy', and much subsequent Western philosophy is a response to his writings, which are studied closely to this day...

, and Gottfried Leibniz
Gottfried Leibniz
Gottfried Wilhelm Leibniz was a German philosopher and mathematician. He wrote in different languages, primarily in Latin , French and German ....

) considered God as logically necessary being, Richard Swinburne argued for factual necessity, and Alvin Plantinga
Alvin Plantinga
Alvin Carl Plantinga is an American analytic philosopher and the emeritus John A. O'Brien Professor of Philosophy at the University of Notre Dame. He is known for his work in philosophy of religion, epistemology, metaphysics, and Christian apologetics...

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.
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