In a letter to James Maury on June 16, 1815, Jefferson discussed his views of relations with England and other nations. Then he reflected on Maury’s bathing habits and the relationship to his own and his generally good health:
Your practice of the cold bath thrice a week during the winter, and at the age of 70.2 is a bold one, which I should not, a priori, have pronounced salutary. but all theory must yield to experience, and every constitution has it’s own laws. I have for 50. years bathed my feet in cold water every morning (as you mention) and having been remarkably exempted from colds (not having had one in every 7. years of my life on an average) I have supposed it might be ascribed to that practice. when we see two facts accompanying one another for a long time, we are apt to suppose them related as cause and effect.
Even Jefferson was subject to illusory correlations. Maury was a former teacher of the young Jefferson and an Anglican minister.