Full Idea
I observe almost no individual effect without immediately knowing that it can be deduced in many different ways, ..and I know of no way to resolve this but by experiments such that the results are different according to different explanations.
Gist of Idea
Only experiments can settle disagreements between rival explanations
Source
René Descartes (A Discourse on Method [1637], §6.65)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.81