 Lewis combined the identification of properties with classes with a standard mathematical theory of sets and classes (Lewis  1991),  on which some things—such as the classes—are too numerous to form a class. His view thus provides for such properties as being grue, but provides no good candidate referents for terms like ‘being a property’, ‘being instantiated by all triangles’, ‘identity’, ‘being more natural than’, etc. Intuitive claims like ‘identity is more natural than being as tall as’ are thus hard to accommodate.