Full Idea
For us, as pure mathematicians, the laws of motion and the law of gravitation are not properly laws at all, but parts of the definition of a certain kind of matter.
Gist of Idea
The laws of motion and gravitation are just parts of the definition of a kind of matter
Source
Bertrand Russell (The Principles of Mathematics [1903], §459)
Book Reference
Russell,Bertrand: 'Principles of Mathematics' [Routledge 1992], p.485
A Reaction
The 'certain kind of matter' is that which has 'mass'. Since these are paradigm cases of supposed laws, this is the beginning of the end for real laws of nature, and good riddance say I. See Mumford on this.