 Two reasons can be given for believing that being a law does not depend on any necessary connection between properties. The first reason is the conceivability of it being a law in one possible world that all Fs are Gs even though there is another world with an F that is not G. The second is that there are laws that can only be discovered in an a posteriori manner. If necessity is always associated with laws of nature, then it is not clear why scientists cannot always get by with a priori methods. Naturally, these two reasons are often challenged. The necessitarians argue that conceivability is not a guide to possibility. They also appeal to Saul Kripke’s (1972) arguments meant to reveal certain a posteriori necessary truths in order to argue that the a posteriori nature of some laws does not prevent their lawhood from requiring a necessary connection between properties. In further support of their own view, the necessitarians argue that their position is a consequence of their favored theory of dispositions, according to which dispositions have their causal powers essentially. So, for example, on this theory, charge has as part of its essence the power to repel like charges. Laws, then, are entailed by the essences of dispositions (cf., Bird 2005, 356). As necessitarians see it, it is also a virtue of their position that they can explain why laws are counterfactual-supporting; they support counterfactuals in the same way that other necessary truths do (Swoyer 1982, 209; Fales 1990, 85–87).