In the fields of observation, chance favors only the prepared mind.

[Dans les champs de l’observation, le hasard ne favorise que les espirits préparés.]

louis pasteur
Louis Pasteur (1822-1895) French chemist, pharmacist, microbiologist
Speech, as new Dean of Science, University of Lille, France (1854-12-07)

Often misattributed to Ansel Adams. It was, though, Adams' favorite aphorism, which he usually paraphrased as to "Chance favors the prepared mind."