 We attempt to motivate this theory (in terms of the cumulative hierarchy). Think of classes as collections which merely exist potentially. The sets are those classes which actually get constructed. Extensionality for classes seems unproblematic. All collections of the actual sets could have been constructed by constructing one more stage of the cumulative hierarchy: this justifies class comprehension. Elements of actual sets are actual sets; subcollections of actual sets are actual sets; these do not seem problematic. Finally, we assert that any collection of classes which is defined without reference to the realm of actual sets, which is defined in terms of specific objects which are actual, and which turns out only to contain actual elements is actual. When one gets one’s mind around this last assertion, it can seem reasonable. A particular thing to note about such a definition is that it is “absolute”: the collection of all actual sets is a proper class and not itself an actual set, because we are not committed to stopping the construction of actual sets at any particular point; but the elements of a collection satisfying the conditions of set comprehension do not depend on how many potential collections we make actual (this is why the actuality predicate is not allowed to appear in the “defining” formula).