Editing Subjunctive possibility (section)

== Types of subjunctive possibility ==
There are several different types of subjunctive modality, which can be classified as broader or more narrow than one another depending on how restrictive the rules for what counts as "possible" are. Some of the most commonly discussed are:

* '''[[Logical possibility]]''' is usually considered the broadest sort of possibility; a proposition is said to be logically possible if there is no logical contradiction involved in its being true. "[[Dick Cheney]] is a bachelor" is logically possible, though in fact false; most philosophers have thought that statements like "If I flap my arms very hard, I will fly" are ''logically'' possible, although they are ''nomologically'' impossible. "[[Dick Cheney]] is a married bachelor," on the other hand, is ''logically impossible''; anyone who is a bachelor is ''therefore'' not married, so this proposition is logically self-contradictory (though the sentence isn't, because it is logically possible for "bachelor" to mean "married man").
* '''[[Metaphysical]] possibility''' is either equivalent to logical possibility or narrower than it (what a philosopher thinks the relationship between the two is depends, in part, on the philosopher's view of [[logic]]). Some philosophers have held that ''discovered identities'' such as [[Saul Kripke|Kripke]]'s "Water is H<sub>2</sub>O" are metaphysically necessary but not [[logical truth|logically necessary]] (they would claim that there is no formal contradiction involved in "Water is ''not'' H<sub>2</sub>O" even though it turns out to be metaphysically impossible).
* '''[[Nomological]] possibility''' is ''possibility under the actual laws of nature''. Most philosophers since [[David Hume]] have held that the laws of nature are ''metaphysically contingent''—that there could have been different natural laws than the ones that actually obtain. If so, then it would not be ''logically'' or ''metaphysically'' impossible, for example, for you to travel to [[Alpha Centauri]] in one day; it would just have to be the case that you could travel faster than the [[speed of light]]. But of course there is an important sense in which this is ''not'' possible; ''given'' that the laws of nature are what they are, there is no way that you could do it. (Some philosophers, such as [[Sydney Shoemaker]] {{Citation needed|date=December 2008}}, have argued that the laws of nature are in fact ''necessary'', not contingent; if so, then nomological possibility is equivalent to metaphysical possibility.)
* '''Temporal possibility''' is ''possibility given the actual history of the world''. [[David Lewis (philosopher)|David Lewis]] ''could have'' chosen to take his degree in Accounting rather than Philosophy; but there is an important sense in which he ''cannot now''. The "could have" expresses the fact that there is no logical, metaphysical, or even nomological impossibility involved in Lewis's having a degree in Economics instead of Philosophy; the "cannot now" expresses the fact that that possibility is no longer open to becoming actual, given that the past is as it actually is.

Similarly David Lewis could have taken a degree in Economics but not in, say, Aviation (because it was not taught at Harvard) or Cognitive Neuroscience (because the so-called 'conceptual space' for such a major did not exist). There is some debate whether this final type of possibility in fact constitutes a type of possibility distinct from Temporal, and is sometimes called Historical Possibility by thinkers like [[Ian Hacking]].