I think we can agree to disagree here. What about a language like Perl, where any reference (not just a reference to a hash) can be blessed and be used to disp

I think we can agree to disagree here. What about a language like Perl, where any reference (not just a reference to a hash) can be blessed and be used to dispatch methods? Would you say OO Perl is not OO? (I think you could argue either way). Personally I think CLOS is a very interesting mode of doing OO. You could always define define multimethods based only on the first argument. Some parts of CLOS (method interceptors) have also made it to Java in forms of AOP (and in fact the author of AspectJ is one of the authors of CLOS) -- which provides "magic" for Java frameworks such as Guice. While I also hate to Biblethump, but Alan Kay has also spoken very favourably of CLOS despite CLOS not being message passing. If you prefer, you can think of CLOS as a "meta OO" which lets you implement different OO behaviour including message passing. Back to Clojure deftype and defrecord do provide encapsulation, by the way: in types with :mutable-unsyncyronized fields can only be accessed via self/this pointer. I do agree that Clojure's protocols/multimethods aren't a full blown CLOS, but they could (sensibly) be called a form of OO (even if it's a form OO that differs greatly from Java and C++)