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In music, modality is the subject concerning certain diatonic scales known as "modes" (e.g., Ionian). See Musical mode.


In language, modality is the subject concerning so-called "modal verbs" like can, must, and should, that are customarily used to modify the meaning of other verbs (which in turn tend to take the infinitive). Modal verbs express possibility (and impossibility, necessity, contingency, etc.), permissibility (and obligation, proscription, etc.), probability (and improbability, etc.).


In philosophy, modality is the subject concerning necessity, contingency[?], possibility, impossibility, actuality, and related predicates. The claim "'2+2=4' is necessary" is a modal claim, as is the claim "'Bigfoot exists' is possible". Necessary propositions either couldn't have been true or couldn't have been otherwise -- perhaps logical and mathematical propositions qualify. Contingent propositions could have been true, but also could have been false -- perhaps "Jupiter exists" qualifies. Possible propositions could have been true -- they include necessary and contingent propositions. Impossible propositions couldn't have been true -- perhaps self-contradictory claims qualify.

Modal claims are to be distinguished from similar-sounding epistemic claims. When a philosopher claims that Bigfoot possibly exists, he probably does not mean "Bigfoot might actually exist, for all I know". Rather, he is making a metaphysical claim concerning ways the world could have been, a substantive claim with apparent ontological commitments. 'Epistemic possibility', on the other hand, just traces the confines of our knowledge. "It is possible that p" may be glossed as "I (or we humans) don't know that p is false". It is a claim about those matters about which we have no knowledge one way or the other. When philosophers say "possible", they usually mean the former. An illustration: Someone asks you if 54 squared is 2926 and you stammer, "I don't know, I suppose it's possible". This is 'for all we know' possibility. For, as it turns out 54 squared is 2916 -- and it is metaphysically impossible for it to have been otherwise (say, 2926).

How to best interpret modal claims is a live issue for metaphysicians. Sometimes modal concepts are cashed out in terms of a "possible worlds idiom", which would translate the claim about Bigfoot as "There is some possible world in which Bigfoot exists". To maintain that Bigfoot's existence is possible, but not actual, one could say, "There is some possible world in which Bigfoot exists; but in the actual world, Bigfoot does not exist".

This idiom still leaves unclear what we are committing ourselves to when we make modal claims. Are we really alleging the existence of possible worlds, every bit as real as our actual world, just not actual? Renowned philosopher David K. Lewis[?] infamously bit the bullet and said yes, possible worlds are as real as our own. This position is called "modal realism[?]". Unsurprisingly, most philosophers are unwilling to sign on to this particular doctrine, seeking alternate ways to paraphrase away the apparent ontological commitments implied by our modal claims.

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