Symbols and search in humans and machines

This chapter examines two major contributions made by Herbert Simon, with his collaborator Allen Newell, to cognitive psychology and artificial intelligence. The first is the theory of physical symbol systems, which introduced the notion of symbol structures and proposed computational mechanisms that manipulate them. The second is the theory of heuristic search, which characterizes problem solving as guided exploration through a space of symbol structures that includes solutions. In each case, I review the structures and processes that the theory postulates, along with hypotheses about the intelligent behavior it supports. In addition, I consider limitations of the frameworks and extensions that address them, as well as alternative accounts of the target phenomena. These profound theories offer deep insights into the computational character of intelligence and the roles played by symbols and search in human and machine cognition.