OOP is defined primarily by its ability to abstract and encapsulate data (and state) in objects. I cannot help but thinking this is a tautology: 'Programming w

OOP is defined primarily by its ability to abstract and encapsulate data (and state) in objects. I cannot help but thinking this is a tautology: 'Programming with objects is defined by putting data in an objects.' What is an object? How does it defer from other data structures where its members are hidden to the outside world? I am not arguing that OOP does not exist, just that one aspect that has always been touted as one of its greatest advantages, also exists in most non-OO languages. The second defining aspect of OOP is messaging (goes along with encapsulation); Alan Kay has more to say about that (just check out Message Passign on wiki). The third is probably dynamic dispatch and polymorphism; not all OOP languages have that. Is message passing without dynamic dispatch and polymorphism really message passing? (In contrast to a function call that has a structure as an implicit argument?) there are other ways to encapsulate state; e.g., as monads A data type is a monad if a function is provided that defines how computations of that data type can be combined[1], and that function obeys the monad laws. A monad can be used to combine effectful computations (IO, ST) or threading state through function calls (State). But e.g. the list, Maybe, or Either monads have little to do with state. [1] Excluding 'return' for brevity ;).