The Actor conceptional model had not yet been developed when SmallTalk-72 was designed following on SmallTalk-71. Later versions of SmallTalk after SmallTalk-72

The Actor conceptional model had not yet been developed when SmallTalk-72 was designed following on SmallTalk-71. Later versions of SmallTalk after SmallTalk-72 adopted the Simula object model. Actors were developed for concurrency, which was entirely lacking from Simula. Also, Simula had procedures attached to the class hierarchy instead of Actor message handlers, which initially seemed like a subtle distinction that grew in importance.