 Contingentism has often been defended by pointing out that the laws of nature can be known only a posteriori, and that their negations are conceivable (see Sidelle 2002). Necessitarians may reply to the first point that Kripke’s work has given us reasons for thinking that a posteriori truths can be metaphysically necessary (see  section 1.1).  In response to the second point, they may grant that the negation of a law is conceivable, but deny that conceivability is a good guide to possibility (see the entry  epistemology of modality).  Alternatively, they may deny that we can really conceive of a situation in which, say, bodies violate the law of gravitation. What we can conceive of is a situation in which objects move in ways that appear to violate the law. But that situation cannot be correctly be described as involving objects with mass. Rather, the objects in the imagined situation have a different property that is very similar to mass (call it ‘schmass’) but which is governed by slightly different laws. Contingentists may reply that the non-existence of schmass (or the non-existence of objects that move in the way imagined) is itself a law, so that we have, after all, conceived of a situation where one of the actual laws fails (see Fine 2002).