A paper by Richard Westfall entitled 'Circular Motion in Seventeenth-Century Mechanics', begins with the following statement.
One prominent contemporary school of history and philosophy of science holds that the principle of inertia can only be understood as a convention that defines uniform rectilinear motion as a natural state which requires no causal explanation. 1
Westfall is certainly right in thinking he has here expressed something recognizable as a (or possibly even the) received view about the law of inertia. 2 1 do not think that the principle of inertia should be understood as a convention, but with that proviso, I accept the position in question, which I shall call the Received View.
Implicit in the Received View is the general claim that natural states require no causal explanation. For the force of the Received View is not that uniform rectilinear motion is a natural state which, as it happens, requires no causal explanation, but rather that uniform rectilinear motion is a natural state and therefore requires no causal explanation. Intuitively, the idea is that a natural state is what would obtain were no causes operafive at all, and hence causes need only be cited in accounting for deviations from natural states. So it seems that the Received View is best understood as dividing into two separate theses: (i) natural states require no causal explanation, and (ii) uniform rectilinear motion is a natural state. My purpose will be to try to clarify these two theses.
II
It must be admitted immediately that there are senses in which it is quite correct to say that states are causally explained. First, there is a sense in which one is said to have causally explained a state if one has explained