J[ohn ]W[atkins]: “Karl, you are dishonest. You hate criticism.”

K[arl ]R[aimund ]P[opper]: “You are dishonest. Your statement refers to my state of mind; it is irrefutable. Only dishonest people raise irrefutable criticism.”

Imre Lakatos, Paul Feyerabend, and Matteo Motterlini, For and against method: including Lakatos's lectures on scientific method and the Lakatos-Feyerabend correspondence, Chicago, 1999, p. 189