> My definition of an object is "an ADT that gets to 'decide', using private runtime state, what to do in response to any/all attempts to interact with it." Whi

> My definition of an object is "an ADT that gets to 'decide', using private runtime state, what to do in response to any/all attempts to interact with it." Which makes for a very simple test: if something is an object, you should be able to send it a message that makes it change its mind about what messages it accepts after that. I'm not sure these are equivalent criterion. In Java and C++, the "dispatch matrix" is defined at compile time, and it is stored at runtime, and the object itself does determine the contents of that matrix. So these languages do clearly meet your first criterion. But also, clearly, not the second. I guess the objection you might raise is "any/all messages", with "modify your dispatch matrix" being a reasonable message. But there will always be some message that cannot be implemented by the object; we can certainly code up some Goedelian thing inside of a core OO calculus. But for any concrete OO language, something more reasonable than that probable exists. So then the question is where -- not if -- we draw the line on "any/all messages". And I'm not sure that drawing that line south of Ruby but north of Java makes a lot of sense.