Theories of Mind as Domain-Specific Languages of Thought

What kind of thing is a ``theory of mind''? We propose to formalize theories of mind as domain-specialized programming languages, which can be used to reason about the mental states of other agents. Our proposal builds on the longstanding idea that thinking is akin to programming in an internal ``language of thought.'' But rather than positing a single general-purpose language for all of thought, we posit a collection of domain-specific languages of thought, whose respective syntaxes and semantices encode the theories of various domains. We make this idea concrete in the domain of intuitive psychology using a real-world programming language called memo, which is specialized for theory-of-mind reasoning via domain-specialized syntactic constructs like ``knows,'' ``wants,'' and ``thinks.'' In our view, using one's theory-of-mind to think about a social situation is analogous to using memo to write a program that models that situation. Through a series of worked examples, we show how formally representing a theory of mind as a memo-like programming language can lend insight into a variety of theoretical issues in theory-of-mind research: (1) how a single, fixed intuitive theory can be used to reason about social situations with unbounded structural variation; (2) how a modular, domain-specialized, and informationally-encapsulated system for social cognition might interface with external world knowledge, as well as other cognitive systems like intuitive physics; and (3) how different theories of mind, such as those of children and adults, differ, what exactly is gained as theories grow over the course of development, and how that growth might occur.