What is originality? Undetected plagiarism. This is probably itself a plagiarism, but I cannot remember who said it before me. If originality means thinking for oneself, and not thinking differently from other people, a man does not forfeit his claim to it by saying things which have occurred to others.

William Ralph Inge (1860-1954) English prelate [Dean Inge]

London Evening Standard (1927)

Parallel to this, in James Marchant, ed., Wit and Wisdom of Dean Inge (1927), Inge is cited as saying, "Originality, I fear, is too often only undetected and frequently unconscious plagiarism."

The sentiment is, appropriately, not original with Inge; see here for more discussion and earlier uses.