books.google.com - Pure Mathematics is the class of all propositions of the form p implies q, where p and q are propositions containing one or more variables, the same in the two propositions, and neither p nor q contains any constants except logical constants. And logical constants are all notions definable in terms of...http://books.google.com/books/about/The_Principles_of_Mathematics.html?id=Yzcf8RFQPmUC&q=class+of+classes&utm_source=gb-gplus-shareThe Principles of Mathematics