I used to say to our audiences: "It is difficult to get a man to understand something, when his salary depends upon his not understanding it!"

Upton Sinclair "I, Candidate for Governor: And How I Got Licked" (1935), ISBN 0-520-08198-6; repr. University of California Press, 1994, p. 109.

Half remembered this the other day but thought it was from Ambrose Bierce.