It is hard to believe that a man is telling you the truth when you know you would lie if you were in his place.

H. L. Mencken (1880-1956) American writer and journalist [Henry Lewis Mencken]

A Little Book in C Major, ch. 2, § 15 (1916)
    (Source)

Variants:

CONFIDENCE. The feeling that makes one believe a man, even when one knows that one would lie in his place.
[A Book of Burlesques, "The Jazz Webster" (1924)]