 A first stab at the notion might use what we now call metaphysical necessity. Perhaps an argument is valid if it is (metaphysically) impossible for the premises to be true and the conclusion to be untrue, valid if—holding fixed the interpretations of premises and conclusion—in every possible world in which the premises hold, so does the conclusion. This constraint is plausibly thought to be a necessary condition for logical consequence (if it could be that the premises are true and the conclusion isn’t, then there is no doubt that the conclusion does not follow from the premises); however, on most accounts of logical consequence, it is not a sufficient condition for validity. Many admit the existence of a posteriori necessities, such as the claim that water is H\(_2\)O. If that claim is necessary, then the argument: