 The introduction of Greek letters for classes in ∗20 and the use of “Roman letters” R, S, …in ∗21 for relations, marks a change in the notation used in PM. After ∗21 the letters \(\phi, \psi, \ldots\) rarely appear. As Quine remarks in his study of the logic of Whitehead and Russell, it would seem that after a certain point the body of PM makes use of extensional higher-order logic in a simple theory of types: