Full Idea
The physicist can never subject an isolated hypothesis to experimental test, but only a whole group of hypotheses; when the experiment is in disagreement with his predictions ...it does not designate which one should be changed.
Gist of Idea
Experiments only test groups of hypotheses, and can't show which one is wrong
Source
Pierre Duhem (The Aim and Structure of Physical Theory [1906], p.187), quoted by Penelope Maddy - Naturalism in Mathematics II.2
Book Reference
Maddy,Penelope: 'Naturalism in Mathematics' [OUP 2000], p.101
A Reaction
This is the idea frequently invoked by Quine, in support of his holistic view of scientific knowledge (along with Neurath's Boat).