the motivations matter. I think Prof. Hewitt was looking for something that could defeat the church-turing hypothesis. The erlang creators cared about fault-tolerance. So other Actor systems out there have different design lineage which means different low-level design choices, and in many ways, they aren't quite the same in ways that are subtle, hard to see, but still quite important to the ergonomics of the developer or maintainer who is in the weeds.