necessity

A proposition is necessary if it could not have been false. We can contemplate various possibilities describing how things might have been but are not; if in all these possibilities a proposition is true, then it is true in all possible worlds or true of necessity. See also necessary/contingent truths, modality, modal logic.

The fundamental explanation of the necessity of a proposition is controversial, and some have doubted whether the category is a useful aid to thought (see Quine ). A relatively clear class of necessary propositions would be those that are analytic (see analytic/synthetic ) since they seem ‘trivially’ true. More substantive problems arise with apparently synthetic necessary truths: classical examples include the proposition that every number has a successor, or nothing can appear red and green all over at the same time. The possibility of synthetic a priori truth is the basic problem faced by Kant in The Critique of Pure Reason . conventionalist theories suggest that we deem certain propositions to be necessary by linguistic convention. Other theories stress the imaginative block we encounter when we try to imagine the possibility of such propositions being true. A category of particular interest recently has been that of propositions which are ‘metaphysically necessary’, yet not in any sense a priori : the proposition that heat in a gas is motion of molecules, or that water is H₂O, for example.

A rather different set of philosophical problems also arises with the idea of the necessary unfolding of events in time; in many philosophies the march of events has been seen as inexorable, or pre-ordained, and this is held to have consequences for how we react to problems of choice, or how we think of what has already happened. See also free will, laws of nature . The cluster of considerations focusing on the cosmological argument seek to show that if anything exists contingently, then something must exist of necessity.