Thinking as a Production System

Since their birth (ca. late 1960s), production systems have been developed as a formal tool not only for describing but for explaining how humans think. Indeed, " to advance our understanding of how humans think " is the stated goal of Newell and Simon's classic (1972) book Human Problem Solving, where the first body of work on production-system models of human thought was presented. The main goal for production systems in psychological research has changed little in the intervening years, and yet the state of the art has advanced dramatically. The aim of the current chapter is to present a contemporary production-systems approach to open questions in problem solving, reasoning, analogy, and language. We will highlight the ways in which today's production systems allow for more flexibility, stochasticity, and sensitivity than their predecessors. Besides demonstrating that production systems can offer insight into current questions and add to our understanding of human thinking, we will discuss our view of production systems in future research. A production system is a set of production rules – each of which represents a contingency for action – and a set of mechanisms for matching and applying production rules. Because the production rule is the fundamental unit of this formalism, it is worth giving a few examples. Table 1 presents four sample production rules written in English. Note that each is divided into two parts by the word " then " : the first part of each production rule (before the " then ") specifies the conditions under which that production rule is applicable, and the second part specifies the actions to be applied. Conditions may reflect an aspect of the external world (e.g., it is dark) or an internal, mental state (e.g., my current goal is to get to a particular location, or I can retrieve a 3 particular fact). Likewise, actions may transform a feature in the real world (e.g., flip the light switch) or an internal, mental state (e.g., change my current goal, or add a fact to memory). To operate, a production system requires a dynamic memory that represents the current state of the system and is used to match against production rules' conditions. For example, when dynamic memory includes the goal " to get to San Francisco, " the second production rule in Table 1 would match for someone in Pittsburgh, Pennsylvania. This pattern matching of production rules to dynamic memory …