Editing Subjunctive possibility (section)

== Subjunctive possibility and other modalities ==
Subjunctive possibility is contrasted with (among other things) [[epistemic possibility]] (which deals with how the world ''may'' be, ''for all we know'') and [[Deontological ethics|deontic possibility]] (which deals with how the world ''ought'' to be). {{further|Counterfactual conditional}}

=== Epistemic possibility ===
The contrast with epistemic possibility is especially important to draw, since in ordinary language the same phrases ("it's possible," "it can't be", "it must be") are often used to express either sort of possibility. But they are not the same. We do not ''know'' whether [[Goldbach's conjecture]] is true or not (no-one has come up with a proof yet); so it is (epistemically) ''possible that'' it is true and it is (epistemically) ''possible that'' it is false. But if it ''is'', in fact, provably true (as it may be, for all we know), then it would have to be (subjunctively) ''necessarily'' true; what being provable ''means'' is that it would not be (logically) ''possible for'' it to be false. Similarly, it might not be at all (epistemically) ''possible that'' it is raining outside—we might ''know'' beyond a shadow of a doubt that it is not—but that would hardly mean that it is (subjunctively) ''impossible for'' it to rain outside. This point is also made by [[Norman Swartz]] and Raymond Bradley.<ref>Norman Swartz and Raymond Bradley, ''Possible Worlds — An Introduction to logic and its philosophy'', 1979, Hackett Publishing company, inc., {{ISBN|0-631-16130-9}}. pp. 329–330</ref>

=== Deontic possibility ===
There is some overlap in language between subjunctive possibilities and deontic possibilities: for example, we sometimes use the statement "You can/cannot do that" to express (i) what it is or is not subjunctively possible for you to do, and we sometimes use it to express (ii) what it would or would not be right for you to do. The two are less likely to be confused in ordinary language than subjunctive and epistemic possibility as there are some important differences in the logic of subjunctive modalities and deontic modalities. In particular, subjunctive necessity entails truth: if people logically must such and such, then you can infer that they actually do it. But in this non-ideal world, a deontic ‘must’ does not carry the moral certitude that people morally must do such and such.