Morality is the theory that every human act must be either right or wrong, and that 99 per cent. of them are wrong.

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

A Little Book in C Major, ch. 2, § 4 (1916)
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Variants:

MORALITY. The theory that every human act must be either right or wrong, and that 99% of them are wrong.
[A Book of Burlesques, "The Jazz Webster" (1924)]

Morality is the theory that every human act must be either right or wrong, and that 99% of them are wrong.
[Chrestomathy, ch. 30 "Sententiae" (1949)]