I agree with the general conclusion (as I understand it) that mutable objects aren't a good way of modeling state, but the case has been made much more strongly

I agree with the general conclusion (as I understand it) that mutable objects aren't a good way of modeling state, but the case has been made much more strongly and more clearly elsewhere, such as here: http://clojure.blip.tv/file/812787/ http://clojure.blip.tv/file/812787/ You can do state and concurrency correctly in an OO language with locks, but it isn't practical. It gets exponentially more difficult as a project grows, and realistically any human being is going to end up with something that only almost always works correctly, with the occasional failures that are left over almost impossible to find.