The research paradigm invented by Allen Newell and Herbert A. Simon in the late 1950s dominated the study of problem solving for more than three decades. But in the early 1990s, problem solving ceased to drive research on complex cognition. As part of this decline, Newell and Simon's most innovative research practices -especially their method for inducing subjects' strategies from verbal protocols -were abandoned. In this essay, I summarize Newell and Simon's theoretical and methodological innovations and explain why their strategy identification method did not become a standard research tool. I argue that the method lacked a systematic way to aggregate data, and that Newell and Simon's search for general problem solving strategies failed. Paradoxically, the theoretical vision that led them to search elsewhere for general principles led researchers away from studies of complex problem solving. Newell and Simon's main enduring contribution is the theory that people solve problems via heuristic search through a problem space. This theory remains the centerpiece of our understanding of how people solve unfamiliar problems, but it is seriously incomplete. In the early 1970s, Newell and Simon suggested that the field should focus on the question where problem spaces and search strategies come from. I propose a breakdown of this overarching question into five specific research questions. Principled answers to those questions would expand the theory of heuristic search into a more complete theory of human problem solving.Keywords action retrieval, cognitive architecture, evaluation function, goal, heuristic search, methodology, problem perception, problem solving, problem space, search strategy, subgoal, think-aloud 1 This article is a greatly expanded and revised version of a presentation, "Getting to heuristic search", at The Journal of Problem Solving • 102
S. Ohlsson• volume 5, no. 1 A Theory Yet to be AchievedIf the task of psychology is to explain what it is to be human, then the study of problem solving is essential. The ability to solve unfamiliar problems has played a central role in human history via technological invention as well as in other ways, and it separates us from other animals because we are not merely better at it, we are orders of magnitude better at it. Within cognitive psychology, the ability to solve unfamiliar problems has served as a prototypical instance of the 'higher' cognitive processes. It would appear impossible to explain problem solving with associationistic theories, but the anti-associationists were on the defensive until the 1950s because they lacked a clearly articulated alternative. This situation changed in the second half of the 1950s when Allen Newell, Herbert A. Simon, and their co-workers and students launched a novel paradigm for the study of problem solving, including an empirical but non-experimental methodology and a new kind of formal theory. The Newell-Simon paradigm was laid out in painstaking detail in their monumental book, Human problem solving (Newell & ...