What’s the difference between forgery and plagiarism?

“This has been answered clearly by Monroe C. Beardsley: In the case of plagiarism one concerns oneself in ‘passing off another’s work as one’s own’; in the case of forgery, in ‘passing off one’s own work as another’s.'”

— Sándor Radnóti, The Fake: Forgery and Its Place in Art, 1999