Truth is stranger than fiction, but it is because Fiction is obliged to stick to possibilities; Truth isn’t.

Mark Twain (1835-1910) American writer [pseud. of Samuel Clemens]

Following the Equator, ch. 15, epigraph, “Pudd’nhead Wilson’s New Calendar” (1897)
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Sometimes paraphrased, "Why shouldn't truth be stranger than fiction? Fiction, after all, has to make sense." More on this quotation and its variants here.

See also Byron.