It is difficult to get a man to understand something, when his salary depends upon his not understanding it!

Upton Sinclair (1878-1968) American writer, journalist, activist, politician

I, Candidate for Governor: And How I Got Licked, ch. 20 (1935)
    (Source)

A regular comment of his on the campaign trail. The wording is Sinclair's, though there are earlier references with the same sentiment (see here for more discussion).

Often misattributed to H. L. Mencken. (e.g., "Never argue with a man whose job depends on not being convinced") though not found in his work.