Message-passing was the only means of interaction. An actor could have arbitrarily many acquaintances that is, it could “know about” (in Hewitt’s language) other actors and send them messages or send acquaintances as (parts of) messages. The objects were called actors, and the messages themselves were also actors.
Hewitt’s model was object-oriented (and influenced by Smalltalk) every object was a computationally active entity capable of receiving and reacting to messages. The dialect of Lisp known as Scheme was originally an attempt by Gerald Jay Sussman and Guy Steele during Autumn 1975 to explicate for themselves some aspects of Carl Hewitt’s theory of actors as a model of computation. In their paper on the evolution of Lisp, Richard Gabriel and Guy Steele explain the origin of Scheme as follows: Owing to the minimalist specification, there is no standard syntax for creating structures with named fields, or for doing object oriented programming, but many individual implementations have such features. Scheme uses lists as the primary data structure, but also has good support for arrays. Scheme also supports garbage collection of unreferenced data. It was also one of the first programming languages to support explicit continuations. Scheme was the first dialect of Lisp to use lexical variable scoping (aka static scoping, as opposed to dynamic variable scoping) exclusively. For example, the main mechanism for governing control flow is tail recursion. Therefore, Scheme provides as few primitive notions as possible, and where this is practical in an implementation, tends to let everything else be provided by Programming Libraries that are built on top of them. Its goal is not to pile feature upon feature, but to remove weaknesses and restrictions that make new features appear necessary. Scheme's philosophy is unashamedly minimalist. Minor implementation details tend to differ slightly, so sometimes Scheme is referred to as a family of closely related programming languages.
Steele and Gerald Jay Sussman in the 1970s initially as an attempt to understand the Actor model and introduced to the academic world via a series of papers now referred to as Sussman and Steele's Lambda Papers.
Scheme is a functional programming language and a dialect of Lisp. The Knights of the Lambda Calculus' recursive emblem celebrates Scheme's theoretical foundation, the lambda calculus.