 The remainder of ∗20 consists of theorems proving that the theorems of quantificational logic developed in ∗10 apply as well to expressions about classes, with the “Greek” variables \(\alpha, \beta, \ldots\) in the place of individual variables \(x, y, \ldots\). Because formulas with Greek variables look and behave the same as individual variables with respect to quantificational logic, it is possible to overlook the interaction of the theory of classes with the theory of types. As Gödel points out in the passage quoted above (Gödel 1944 [1951: 126]), the “contextual definitions” of class variables \(\alpha\), \(\beta\), etc., does not specify the elimination of class abstracts from all possible contexts, and in particular those that talk about classes. Linsky (2004) argues that PM has no notation for classes of propositional functions to distinguish them from classes of classes, although one could be added. This is another indication of the turn in PM after the intial sections (up to ∗21) to an extensional system of classes and relations.