CONSCIENCE: the inner voice which warns us that someone may be looking.
H. L. Mencken (1880-1956) American writer and journalist [Henry Lewis Mencken]
A Little Book in C Major, ch. 4, § 12 (1916)
(Source)
Reprinted in A Mencken Chrestomathy, ch. 30 "Sententiae," "The Mind of Men" (1949), as "Conscience is the inner voice ...."