In the Metaphysical Necessity Theory, Laws of Nature are the 'principles' which govern the natural phenomena of the world. In other words, the natural world 'obeys' the Laws of Nature. Although Ayer does a pretty decent job of attempting to buckle down the RNT with his ERT, he seems to have conflated the difference between `Why' we believe that something is a law, and `What' indeed is a law.1 What is important here is that there is a set of laws of nature that are metaphysically necessary. These laws are the ones that attribute essential properties to natural kinds. In other words, what makes water, always water universally.
Hilary Putnam and Saul Kripke actually put forth the fact that there are essential
properties of items. These essential properties can be known a posteriori,
empirically, but that does not mean that the law is contingent, as Hume pointed
out. Furthermore, that these essential properties are of a microstructure. So
that, for example, H
0 is an essential property of what we commonly
know as water. It is something that by its very nature has two hydrogen atoms
and one oxygen atom in a particular spacial configuration. The natural law (of
kinds) for water would be this very same atomic configuration. Clearly it would
not be the same if it were HO (or hydrogen peroxide, which
is a poision to most biological life.)
Putnam assumes that these essential qualities are to be held across all possible worlds uniformly. ``Thus not only are actual F's all G's, all possible F's are...[emphasis added]''[I, p. 857] More importantly,