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)]