The World as One of a Kind: Natural Necessity and Laws of Nature

Abstract

This world is one of a kind. Some philosophers have maintained that there are many other worlds which are spatially, temporally and causally unrelated to ours. We are not asserting that there are any such disconnected worlds. Nor do we assert that there are none. There is at least one world; and it is a member of a natural kind whether or not there are any others of its kind. If there were any other world, in addition to this one, or instead of this one, then there would be a nontrivial question whether that world was of the same natural kind as ours. We can imagine worlds which would be of the same natural kind as ours; but we can also imagine worlds which would not. Recognition that this world is one of a kind offers a new approach to the question of what a law of nature is. We argue that in general laws of nature are concerned with natural kinds. In some cases laws simply describe the essential properties of natural kinds. Maxwell's equations, for example, describe the essential properties of the electromagnetic field. In the case of other laws, for instance where there are interactions between things of different kinds, the laws stating how they behave are derivable from their essential natures. We hold that this is true even of the most fundamental laws of nature, e.g. the conservation laws, the principles of relativity, and the symmetry principles: they too are concerned with the essential properties of a natural kind. Their concern is with the kind of world this is. This theory of the nature of scientific laws derives from the basic idea that things behave as they do because of what they are made of, how they are made, and what their circumstances are. In so far as the behaviour of a thing depends on what it is made of, it depends on the essential natures of its constituents. In so far as its behaviour depends on how it is made, or on its