John asks:
Is there ANY notion of ‘metaphysical necessity’ that you consider to be defensible ?
Answer by Craig Skinner
Yes, metaphysical necessity as the strictest or strongest grade of necessity: absolute; necessity in virtue of the essences of things.
The grades, in increasing strictness, are:
1. Practical
2. Physical
3. Nomological
4. Logical
5. Metaphysical
Practical necessity refers to things we need to do in ordinary life to ensure our plans go smoothly eg before setting off to drive to Italy it’s necessary to check tyres, oil, insurance etc. But we could ignore this need and set off without doing any of it.
Physical necessity refers to something forced on us, not by logic or the laws of nature, but by the limits of our powers eg inability to time travel because we cant make big enough exotic wormholes in space or make a cylinder the mass of a galaxy spinning at half light speed. But maybe one day we will be able to do these things.
Nomological (Greek nomos = law) necessity is forced by the laws of nature eg on Earth a book dropped falls to the ground (law of gravity). But there may be other universes with different laws of nature where such a thing doesnt necessarily happen.
Logical necessity is necessity in virtue of the meaning of words (or symbols) plus the laws of logic eg “all bachelors are unmarried men”, “all red balls are red” are necessarily true. But maybe some other universes are illogical. Indeed, there are some true contradictions in our world eg “This sentence is not true” is both true and untrue.
Metaphysical necessity is absolute necessity, simply must be, no ifs, no buts. Examples are that I am necessarily the child of my specific parents (a child of somebody else couldnt be me); or that water is liquid H2O; or gold is the element with atomic number 79. We can say metaphysical necessity is due to the essences of things (the properties that make a thing the very thing it is). Nothing exists by metaphysical necessity though – any item in the world, or the entire universe itself, might not have existed. Of course we can define God as the being whose existence is his essence, in which case, if God exists, then his existence is metaphysically necessary.
The term “natural necessity” is sometimes used to mean physical, or nomological, or both these necessities, but is confusing and best avoided. Likewise “conceptual necessity”, due to definitions or meanings of words, which I include under logical necessity.
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