 It should be noted that every s-type is also an r-type, namely one that is hereditarily predicative. Thus it might seem that the expressions of the theory of classes are all simply a special case of formulas of the full system of the ramified theory of types. This will be true of the assignment of types to variables, but it must be remembered that the entire formula \(\phi \{x \mid \psi (x) \}\) about a class is by definition