Full Idea
No more causes of natural things should be admitted than are both true and sufficient to explain the phenomena. …For nature does nothing in vain, …and nature is simple and does not indulge in the luxury of superfluous causes.
Gist of Idea
We should admit only enough causes to explain a phenomenon, and no more
Source
Isaac Newton (Principia Mathematica [1687], Bk 3 Rule 1)
Book Reference
Newton,Isaac: 'Philosophical Writings' [CUP 2004], p.87
A Reaction
This emphasises that Ockham's Razor is a rule for physical explanation, and not just one for abstract theories. This is something like Van Fraassen's 'empirical adequacy'.