  For our purposes, classes may be taken as   definable subcollections of the universe of sets.   For example, if  a  is any set, then the class of  all sets which do not contain a   as an element is {x :a  ∉ x}.   In  specifying a class, one may use the first-order language with the  membership symbol and the rest of the syntax from logic, and one may  also use particular sets as parameters, as we have just done. 