The sensible facts are discoverable by our senses, the insensible facts by our thoughts. The invention of hypotheses is therefore nothing more than a mental effort to bring insensible facts into causal relation with sensible facts, and such an effort of correlation is praiseworthy even if it is daring.

In: H. Shapley, H. Wright, and S. Rapport (eds.), Readings in the Physical Sciences, The Reasonableness of Science (p. 22)