Maria Montessori, Gerald Lee Gutek (2004). “The Montessori Method: The Origins of an Educational Innovation : Including an Abridged and Annotated Edition of Maria Montessori's The Montessori Method”, p.114, Rowman & Littlefield
Man and Superman "Maxims for Revolutionists" (1903). A further extension appears in Jacob M. Braude, Speaker's Encyclopedia of Stories, Quotations, and Anecdotes (1955): "Those who can, do; those who can't teach; and those who can't do anything at all, teach the teachers."