A big challenge with OOP is that it was never rigorously defined. Objects originate in Simula-67. OOP derived from objects of Simula by its very name. Simula

A big challenge with OOP is that it was never rigorously defined. Objects originate in Simula-67. OOP derived from objects of Simula by its very name. Simula does not have messages. Instead Simula has virtual procedures attached to classes such that each object is an instance of a class. Furthermore, Simula does not have interfaces. Actors originated in 1973 as the universal abstraction for digital computation including concurrency, which was omitted from Simula and subsequent OOP programming languages. Furthermore, Actors differ from OOP objects in that they are rigorously characterized up to a unique isomorphism. Actors are computationally more powerful than the lambda calculus and Turing Machines.