 The paper of Martin Hugo Löb (1955), augmented by comments of a referee, brought substantial advances on various fronts. First, it introduces the now standard Löb derivability conditions discussed previously in the context of the second incompleteness theorem. Second, it contains Löb’s solution of Henkin’s problem about sentences “expressing their own provability”. Third, it contains a generalization now called “Löb’s Theorem”, but which Löb actually credits to the anonymous referee (who happened to be none other than Henkin himself; the whole story is told in Smoryński 1991.)