Jonathan Rees had said: "Here is an a la carte menu of features or properties that are related to these terms; I have heard OO defined to be many different subsets of this list." The aspects he names: "encapsulation", "protection", "ad hoc polymorphism", "parametric polymorphism", "everything is an object", "all you can do is send a message", "specification inheritance = subtyping", "implementation inheritance/reuse", "sum-of-product-of-function pattern". http://www.paulgraham.com/reesoo.html