. Then
appears as a variable, and must be of some determinate type. All that we know necessarily about the type of
is that it takes arguments of a given type—that it is (say) an
-function. But this, as we have just seen, does not determine its type. If we are to be able (as our fifth requisite demands) to deal with all classes whose members are of the same type as
, we must be able to define all such classes by means of functions of some one type; that is to say, there must be some type of