Known sourcecanonical
"If I am Sophocles, I am not mad; and if I am mad, I am not Sophocles."
Sophocles / Vit. Anon. p. 64 (Plumptre's Trans.).
Bartlett's Familiar Quotations, 9th ed. (Little, Brown, 1905), public domain
Bartlett's Familiar Quotations, 9th ed. (Little, Brown, 1905), public domain