From his commencement address at Yale University,
June 11, 1962:

For the great enemy of truth is very often not the lie–deliberate, contrived, and dishonest–but the myth–persistent, persuasive, and unrealistic. Too often we hold fast to the cliches of our forebears. We subject all facts to a prefabricated set of interpretations. We enjoy the comfort of opinions without the discomfort of thought.