 This raises some controversial issues. For the purposes of this essay, we make the following three assumptions. First, we assume that metaphysical necessity is just as strong as logical necessity. There are, to be sure, metaphysically necessary truths that are not logical truths, such as the truth that water = H20. But metaphysical necessity is just as strong as logical necessity in that the space of metaphysical possibility is exactly the same as the space of logically possibility: the logically possible worlds = the metaphysically possible worlds (see, e.g., McLaughlin 1995; Chalmers 1996; and Jackson 1998). This is not entirely uncontroversial, but little we have to say turns on it. Second, and more importantly in what follows, we assume that at least certain properties that figure in laws of nature do not play their nomic roles essentially, so that it is logically or metaphysically possible for those properties not to figure in the laws in question. This is also controversial, but we will not defend it here. (See Shoemaker 1980, Swoyer 1982, and Ellis 2001 for the opposing view.) Finally, we will assume that whatever is metaphysically necessary is nomologically necessary, but not conversely. (One can hold that there are nomologically necessary truths that are not metaphysically necessary, even if one holds that all nomic properties play their nomic roles essentially; see Fine 2002.)