Full Idea
Newton has a general law about the effects of his forces, ...but there is no corresponding general principle about the causes of such forces.
Gist of Idea
Newton's laws cover the effects of forces, but not their causes
Source
report of Isaac Newton (Principia Mathematica [1687]) by David Papineau - Thinking about Consciousness App 3
Book Reference
Papineau,David: 'Thinking about Consciousness' [OUP 2004], p.238
A Reaction
I'm not sure that Einstein gives a cause of gravity either. This seems to be part of the scientific 'instrumentalist' view of nature, which is incredibly useful but very superficial.