If Fx is true when, and only when, x is determined to be either 4) or some other propositional function extensionally equivalent to (A, then the proposition F4 is of the form which is ordinarily recognized as being about the class determined by )x taken in extension - that is, the class of entities for which 4)x is a true proposition when x is determined to be any one of them.

Now consider a propositional function Fx in which the variable argument x is itself a propositional function.