Full Idea
The causes assigned to natural effects of the same kind must be, so far as possible, the same. For example, the cause of respiration in man and beast.
Gist of Idea
Natural effects of the same kind should be assumed to have the same causes
Source
Isaac Newton (Principia Mathematica [1687], Bk 3 Rule 2)
Book Reference
Newton,Isaac: 'Philosophical Writings' [CUP 2004], p.87
A Reaction
It is impossible to rule out identical effects from differing causes, but explanation gets much more exciting (because wide-ranging) if Newton's rule is assumed.