ABSTRACT
A requirement of an information processing account of human problem solving is that it
includes a mechanism by which people remember which goals and operators have been
evaluated and which still need to be evaluated. Whether the task is the Tower of Hanoii, a
waterjugs problem, a world-wide web search problem or a spatial navigation task, a
person engaged in search examines the consequences of applying an operator to a state by
trying it out and perceiving to which state it, and subsequent operators, lead. At some
point in the future, the person may, through backup, or because of loops, find themselves
in a visited state. Recognition that the state has already been visited and/or that the
operator has already been applied to this state, will in the long-term help prune the search
space and thereby constrain the effort spent on attaining the goal. This constraint has been
used in a number of models of human problem solving (Anderson, 1993; Howes, 1994).
