Excellent video by Dan Ingalls on the Xerox early history of Smalltalk.

In November 1972, Alan Kay gave an inspiring seminar at MIT

on Smalltalk-72. There were ongoing discussions during the weeks after the lecture about the following:

    Dennis and van Horn capabilities 
    Petri nets
    Smalltalk-72
    Simula, which was strangely very different from Smalltalk-72 
    the Internet
    etc.) 

After extensive discussion about limitations of previous

efforts, we realized that

there is a single abstraction that provided the foundation

for all of computation which it was decided to name the

"Actors Abstraction". The Actors Paradigm covers all of

digital computation and consequently is more powerful than

Church/Turing Paradigm.

     Although the use of bitmap displays with Smalltak-72 
     was very impressive, Smalltalk-72 left a great deal to 
     be desired as a programming language.  
     Later Smalltalk designs were completely 
     different because they adopted the Simula class 
     hierarchy instead of the byte stream interpreter of 
     Smalltalk72

Limitations of Smalltalk72 reinforced the decision to not

define the fundamental abstraction of computation using a

programming language. Instead computation should be

defined axiomatically in terms of fundamental concepts including

       Actors
       Computational Events
       Partial order on computational events
       Regions of mutual exclusion
       Event induction as the foundation for proving properties of Actors Systems

See the following on how to axiomatically define computation

using the Actors Abstraction:

https://papers.ssrn.com/abstract=3418003

https://papers.ssrn.com/abstract=3459566

Also, see the following video:

https://www.youtube.com/watch?v=AJP1VL7shiI