Our ability to reflect in a complex way on our lives, to plan and solve problems that arise on a daily basis is the bedrock of thinking behaviour. However, as in all things human, the ways in which we think are many and varied; from solving puzzles in the newspaper to troubleshooting on a car break-down to developing a new theory of quantum fields. Consider a sample of the sorts of things to which we apply the term "thinking".

First, a fragment of Molly Bloom's sleepy thoughts from James Joyce's Ulysses (1922/1960, pp. 871872), about Mrs Riordan:

.God help the world if all the women in the world were her sort down on bathingsuits and lownecks of course nobody wanted her to wear I suppose she was pious because no man would look at her twice I hope I'll never be like her a wonder she didn't want us to cover our faces but she was a welleducated woman certainly and her gabby talk about Mr. Riordan here and Mr. Riordan there I suppose he was glad to get shut of her.

Next, a person (S) answering an experimenter's (E) question about regulating the thermostat on a home-heating system (Kempton, 1986, p. 83):

E: Let's say you're in the house and you're cold.Let's say it's a cold day, you feel cold, you want to do something about it.

S: Oh, what I might do is, I might turn the thing up high to get out, to get a lot of air out fast, then after a little while turn it off or turn it down. E: Un-huh.

S: So there are also, you know, these issues about, um, the rate at which the thing produces heat, the higher the setting is, the more heat that's produced per unit of time, so if you're cold, you want to get warm fast, um, so you turn it up high.

Finally, a protocol of one of the authors adding 457 and 638 aloud:

Eight and seven is fifteen and then you carry one so, one and three is four and five is nine, and six and four is ten, so the final number is.. .one, nought, nine, five; one thousand and ninety-five.

These three samples illustrate several general aspects of thinking. First, all the pieces involve individuals being conscious of their thoughts. Clearly, thinking must involve conscious awareness. However, we tend to be conscious of the products of thinking rather than the processes themselves. For example, we are conscious of taking eight and seven to add, to produce fifteen, but the thought processes responsible for the answer are unconscious and not open to introspection. Furthermore, even when we can introspect on our thoughts, our recollections of them are often inaccurate. Joyce does a good job of reconstructing the character of idle, associative thought in Molly Bloom's internal monologue, but if we interrupted her and asked her to tell us her thoughts from the previous five minutes, little of it would be recalled accurately Similarly, in psychological experiments retrospective recollections of conscious thoughts are often unreliable. In fact, even the use of concurrent protocols taken as thoughts are being produced is only reliable under some conditions (see Ericsson & Simon, 1980, 1984; for more details see Chapter 1).

Second, thinking can vary in the extent to which it is directed (Gilhooly, 1995). At one end of the scale it can be relatively undirected and at the other extreme it can be sharply directed towards a specific goal. Molly Bloom's piece is more undirected relative to the other pieces. On the point of slipping into a dream, she is just letting one thought slide into another. If she has any goal it is a very general and ill-defined one (e.g., reflect on the day's happenings). In the other two pieces, the goal is much clearer and well defined. In the addition example, a specific answer must be provided that is known to be either right or wrong (i.e., the goal is clearly defined and can be evaluated easily). As we shall see, most of the research on thinking has been concerned with relatively well defined, goal-driven situations and, hence, these situations will be the main focus of the chapters in this part of the book (see Gilhooly, 1995, for an exploration of undirected thinking).

Third, the amount and nature of the knowledge used in different thinking tasks can vary enormously. For example, the knowledge required in the addition case is quite limited. It mainly hinges on knowing how to add any number between one and ten and the rule that you carry numbers above ten from one column to the next (see Anderson, 1993, for a production system model of this behaviour). On the other hand, Molly Bloom is using a vast amount of knowledge about the mores of old widows, expectations about what she herself will be like when old, general knowledge about the irony of those who criticise that which they cannot do themselves, and much more besides. Technically, situations that require little knowledge are called knowledge-poor whereas those requiring more knowledge are termed knowledge-rich. Knowledge-rich situations are much harder to characterise because of the amount of knowledge involved and the variety of ways it is used.

In considering the literature on problem solving, in the first half of this chapter, we will concentrate on research that examines puzzle problems, which tend to be knowledge-poor. In the second half of the chapter, we will consider knowledge-rich types of problem solving: typically, this has been investigated by studying the problem-solving behaviour of experts. The puzzles reviewed here differ considerably from everyday, real-world problems. However, as laboratory tasks they have allowed us to make initial inroads on human problem solving, which have fuelled advances in the understanding of more knowledge-rich situations. Later, we will consider the "ecological validity" of these problems and how they differ from real-world problems.

Overview of thinking chapters

Thinking research is reviewed in four chapters (Chapters 14-17) covering: (a) puzzle solving, insight, and skilled thinking, (b) creativity and discovery, (c) deductive reasoning, and (d) judgement and decision making. These divisions reflect the way thinking research has developed historically and fundamental distinctions made in this area. These divisions are unlikely to be reflected in everyday thought, which may involve a complex admixture of thinking styles. However, the subject matter is made more manageable by introducing these divisions. All these types of thinking share the property of being directed towards relatively definite goals.

This chapter is broadly structured along historical lines. So, we begin with a review of early problemsolving research—the Gestalt school— before turning to a treatment of the information-processing theories of problem solving that emerged in the 1950s and 1960s. We also consider how these information-processing theories have tried to account for the findings of early research. Then, we follow the subsequent development of these theories in studies of expertise, before reviewing some cognitive neuropsychological research on thinking. In Chapter 15 we consider aspects of creativity and discovery. Chapter 16 deals with reasoning using the conditional (i.e., if), and Chapter 17 covers the related area of judgement and decision making. Traditionally, these areas have been dealt with in a quite separate fashion from problem solving. Happily, in recent years, there has been much more convergence in the studies of thinking with an accelerated move towards unified theories that might one day be able to accommodate all of these diverse strands.


At the beginning of the 20th century, adherents of the Gestalt school of psychology extended their theories of perception to problem-solving behaviour. These researchers were particularly creative in performing experimental tests of their theories and produced a large corpus of evidence. During the behaviourist period much of this research was re-interpreted in behaviourist terms, even though the basic experimental paradigms remained unchanged (Maltzman, 1955). During much of the 1950s and 1960s this type of problem-solving research became a background activity, although it has been researched actively again more recently, especially when it was re-interpreted in information-processing terms (see e.g., Bowden & Beeman, 1998; Ohlsson, 1984a, 1992; Raaheim, 1974; Weisberg & Suls, 1973).

Gestalt research on problem solving in animals

The work of the Gestalt school of psychology had its origins in problem-solving research on animals. Early associationist and behaviourist psychologists had characterised problem solving as the result of either trial-and-error or the reproduction of previously learned responses (Hull, 1930, 1931; Maltzman, 1955; Thorndike, 1911). Following Lloyd Morgan's (1894) observations of his dog, Thorndike's famous experiments on cats were taken as strong evidence for this view.

Thorndike had placed hungry cats in closed cages within sight of a dish of food, outside the cages. The cage-doors could be opened when a pole inside the cage was hit. Initially, the animals thrashed about and clawed the sides of the cage. Inevitably, at some point, the cat hit the pole inside the cage and opened the door. On repeated trials, when the cat was placed in the cage again, similar energetic behaviour ensued but gradually the animal seemed to learn that hitting the pole opened the cage door. Eventually, when placed in the cage it went to the pole, hit it and escaped. So, new problems were initially solved by trial-and-error behaviour and then accidental solutions were amalgamated into responses that were reproduced when the appropriate stimulus was presented.

One of the founders of the Gestalt school, Wolfgang Köhler, disagreed with this formulation and believed that there was more to animal problem solving than trial-and-error and reproductive responses. The Gestalt psychologists had been fairly successful in showing that perception was something more than mere

A Necker cube which illustrates the perceptual restructuring in which the corner marked "Y" alternates between being at the back and the front of the figure.

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