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)
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Variants:

CONSCIENCE. The inner voice which warns us that someone is looking.
[A Book of Burlesques, "The Jazz Webster" (1924)]

Conscience is the inner voice which warns us that someone may be looking.
[Chrestomathy, ch. 30 "Sententiae" (1949)]