Full Idea
The feeling of certainty is the most difficult to develop. Initially one seeks explanation: if a hypothesis explains many things, we draw the conclusion that it explains everything.
Gist of Idea
If we find a hypothesis that explains many things, we conclude that it explains everything
Source
Friedrich Nietzsche (Unpublished Notebooks 1872-74 [1873], 19 [238])
Book Reference
Nietzsche,Friedrich: 'Unpublished of 'Unfashionable Obs' period (v 11)', ed/tr. Gray,Richard T. [Stanford 1995], p.75
A Reaction
As so often, a wonderful warning from Nietzsche to other philosophers. They love to latch onto a Big Idea, and offer it as the answer to everything (especially, dare I say it, continental philosophers).