 …I do not understand the English language well enough to be able to say definitely that Russell’s theory (Principia Mathematica I, 54ff) agrees with my theory of functions of the first, second, etc. levels. It does seem so. But I do not understand all of it. It is not quite clear to me what Russell intends with his designation \(\phi \bang \hat{x}\). I never know for sure whether he is speaking of a sign or of its content. (Frege 1980: 78)