The Nature of Intelligence

Chapter 7. Overt and Mental Trial and Error

L.L. Thurstone

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1. Overt trial and error
2. Perceptual trial and error
3. Perception as a conditioning process
4. Ideational trial and error
5. Conceptual trial and error


We have considered the primary function of mind to be the intermediary by which the wants of the organism become satisfied. Its basic function is the anticipation of experience, including the overt behaviour and the consequences of behaviour in the satisfactions that the organism is seeking. Consciousness is essentially the anticipation of contact experience and satisfaction in terms of the sensory cues which indicate what the expected experience is going to be like, if it is completely enacted. This equivalence enables the organism to steer its course in preparing adjustments before these adjustments are actually carried out. The development of intelligence is marked by the capacity to select among incomplete and tentative indices of conduct the ones that are to be fruitful, without actually trying these incomplete actions in final overt form.


The lowest form of mental organization, the one in which mentality is, strictly speaking, entirely absent, is that in which every impulse is carried out into behaviour without inhibition or choice. The behaviour is then entirely random. The life-impulses of the organism become automatically random behaviour, and this random behaviour is kept up until satisfaction is attained, when the motive power for the random behaviour disappears and the random restless behaviour ceases. This is what we shall call overt trial and error. The point at which the trial-and-error choices are made is at the terminal end of the circuit. Think of a cat trying to get out of a box without knowing how to operate the latch of the door. Here we have a motive to get out of the box. The motive would normally issue by simply stepping out. Since this does not work, the motive defines itself by random expression, pawing, scratching, and pushing. The motive is to be thought of as a pressure or force that expresses itself in various ways, more or less at random, until the motive suddenly is neutralized and disappears. The particular move that satisfies the motive is said to be the successful one. It is arrived at by random trial and error. Each move is actually executed without noticeable anticipation of consequences. Each failure is objectively recorded in the environment and in time.



The primary function of the sense-organs is to enable the organism to deal with incomplete behaviour, to select among tentative actions before they are completely expressed. The tentative actions are represented by the sense-impressions. It is clear that the distance sense departments afford a control over future action more effectively than the control that is possible by the contact sense-impressions. The visual sense-impression is in effect an index of expected action. Its effectiveness as a means for perceptual trial-and-error choice depends on the fact that in the past history of the organism the visual impression and the contact experience have been experienced together. The visual impression normally precedes the contact experience and hence, on subsequent occasions, the visual impression alone serves to select the action which would finally be taken if the contact experience were allowed to complete itself.

If we think of ourselves in the situation of the cat that is trying to escape from a box, we would discover fewer random overt acts if the urgency of the situation were not too strong. We would look at the walls of the box, and if they are found by their visual appearance to be flimsily constructed, we might push through the wall to freedom. If, on the other hand, the visual cue represents the wall to be made of solid brick, or iron bars, we would not so readily attempt the overt completion of the act by simply pushing ourselves through the wall. The trial-and-error

(115) choice would be made at the incomplete stage of formation at which the expected action is only a visual index of the expected contact experience. If we found a latch on the door we should have some random movements with the latch and some attitudes of visual inspection of the latch. Some of the ideas regarding the manipulation of the latch would be discarded before ever reaching overt expression. The difference between these two extremes is that in one case the random ideas that occur are executed in overt form, whereas in the second case the random ideas are checked long enough to be lived through mentally before being allowed to express themselves in overt adjustment. Both procedures illustrate trial and error. One is overt trial and error while the second is mental trial and error. All thinking is in this sense trial and error which is carried on mentally instead of overtly. The intelligent adjustment is one that is executed only after having been anticipated mentally.


The behaviouristic school of psychology has as one of its fundamental experimental procedures the conditioning process. If a pain stimulus is given to an animal simultaneously with the ringing of a bell, and if this coincidence is repeated a number of times, it is found that the ringings of the bell alone will elicit the same reaction of withdrawal as the normal pain stimulus. The auditory stimulus has then been grafted, as it were, on the reaction which is normally made to the pain stimulus. The reaction to

(116) pain can, by such experimental methods, be obtained from the auditory stimulus in the absence of the pain stimulus by the mere fact that the two stimuli have been closely associated on a number of occasions. This is known as the process of conditioning.

This process of conditioning is, by no means, a special phenomenon. It is basic for the very process of perception itself. The visual impressions of a knife are associated with the contact experience of handling it and using it. The visual reflections from the smooth steel surface are associated with smoothness to the touch. The appearance of the knife and of the edge are associated with the relative ease or difficulty of cutting with the knife. After a number of such experiences the visual impressions alone, without the associated contact experience, serve the same purpose as the combination of the visual and the contact experiences. The visual sense-impression has then acquired meaning in the true sense of the word. The action of using the knife is then facilitated or inhibited in terms of the incomplete form of it which is the visual sense-impression alone. We have then profited by experience. We have learned. We are then able to remove the trial-and-error process from the realm of overt behaviour to the stage at which the behaviour is still incomplete and mental. If it were not for this conditioning process, perception and mentality would have no biological function.



We have described the trial-and-error process as taking place either in overt form or in the form of incomplete mental antecedents of action. By perceptual trial and error we mean the process of selecting from among those perceived possibilities of behaviour that one which means the desired consequences in terms of past identity of the sense-experience and the corresponding contact experience. The impulse may appear in the degree of definition which corresponds to the percept with the exception that the stimulus or sensory cue has not yet been found. When that happens we have ideational trial and error. The impulse is facilitated or it fails to remain identified with the momentary self in accordance with its degree of similarity with the impulses that constitute the self at the moment. If an impulse particularizes itself in such a way that it becomes more and more unlike the desires that constitute the self, there is a corresponding reduction in the urgency or reality of the impulse. If it defines itself with particulars that frustrate the motives of the self, the impulse disappears altogether. The impulse, it must be remembered, is not something apart from the self. The self is made up of these impulses, and they remain vital and urgent factors in determining behaviour only in so far as they retain their identity with the inner self as they complete themselves toward action. The process of adding attributes to the impulse in its course toward expression is entirely fortuitous as far as the self is concerned. It depend

(118) largely on the past experience of the actor and on the flexibility of his mentality. He has himself no control over the particularized forms that his impulses will take. The self consists in the impulses in their universal unlocalized form. The detailed expressions of the impulses do not necessarily represent the true self.

Suppose that an executive is considering the relative advantages of two policies such as promotion to positions of responsibility from within the organization and the selection of outsiders for such positions. The former alternative makes for good morale by placing before the employees an objective. The second alternative gives the opportunity to bring in new talent and the advantage of a wide field in which to choose trained men. To make up one's mind on such a problem is to state the problem in its abstract form, in the form of a policy, and to let this particularize itself in as many ways as possible.

In Fig. 10 we have a diagrammatic representation of the intension-extension attributes of impulses at different stages of the reflex circuit. If an impulse is being the subject of trial-and-error choice while it is still in the form of a universal, there are many variations of behaviour that are possible as detailed expressions of the impulse. If, on the other hand, the impulse does not become focal for trial-and-error choice until it has defined itself closely, there are, of course, only a relatively small number of possible variations in behaviour among which to specify the impulse into conduct. If the impulse be focal for conscious selection at the stage D there would be, in the diagram, twice as many forms of possible behaviour as

Figure 10, Intension and Extention of response

( 119) if the impulse is not conscious until it reaches overt expression in the form of its consequences. If the impulse is rendered focal at the stage B while still a loosely organized universal or concept, there are eight times as many lines of behaviour possible as in overt trial and error. Each of the eight actions may be anticipated mentally and eliminated if they prove to be failures without ever recording the failures on the environment. The numerical relations of the diagram are of course entirely arbitrary for illustrative purposes.

If a boy is asked to construct the arc of a circle of three inches radius so that it will be tangent to two given circles, he may proceed with several degrees of intelligence. He may proceed by the cat's method, sticking the compass into the drawing aimlessly and drawing circles with centres and radii entirely at random until the task is done. This would be overt trial and error without conscious guidance All the failures would be objectively recorded on the environment and none of them would be mentally inhibited.

He might proceed with the degree of intelligence that ordinary vision affords. He would then foresee the absurdity of some of his previous circles, and these absurdities would be abandoned mentally without being recorded on his environment. This would be an example of perceptual intelligence.

He might notice the inevitable common element of the three-inch radius in all successful adjustments and set his compass accordingly for all his trials. This would be an abstraction in that his problem is made focal at an unfinished stage. He would still make several errors before

(120) locating the right centre. If he were to strip the problem of all the details that are not necessary to identify it, he would be dealing with the principles of constructive geometry, and he would locate the correct centre without recording any failures on the drawing. That would be ideational or conceptual intelligence. His trial-and-error process would be taking place with impulses to draw circles while these impulses were still only principles of geometry instead of actual muscular movements with the compass in hand.


I recently heard a discussion of the policies of the League of Nations. The particular topic was the nature of the police power by which the League should maintain its decisions. It was suggested that the League should enforce its decisions by public opinion in all the nations of the League. This was presented as a novel idea because we have hitherto always taken for granted that the two items " court " and " police " are necessary attributes of law and order. The novelty of the proposed solution consists psychologically in that its author stripped the item " police " as a superfluous attribute from his concept of civic order. He allowed his concept to particularize itself along some other route toward practical execution. This route can be represented as a cross-road that bears the sign " international public opinion ". This sign we are now stopping to look at. The road that we have always taken in the past bears the sign " military police power ", but we have been so accustomed to take this

(121) road that we have never even seen the sign. Both roads are expressions of the motive to have civic order. We have never before been conscious at so early and abstract a stage in the expression of this motive. We are in the habit of adding preconsciously the attributes of military police power so that these attributes are already present and taken for granted when the motive becomes focal for conscious selection of further particulars.

We can represent the pre-focal particularization as though we were passing a fork in the road at night while we are asleep. In the morning we wake up and are consulted about the cross-roads further on. The cross-roads that we passed during the night are taken for granted because we know nothing about them. The later cross-roads are, of course, of more restricted significance than the choices that could have been made earlier in the trip. The decision at every fork in the road represents the addition of particulars to the motive in its course toward overt expression. Our location when we wake up represents the status of the motive when it becomes focal. The turns that are made after we wake up represent the control of foresight or intelligence. At every turn we attempt to live through imaginally what we may expect ahead of us. We inhibit our progress until we have done this, and we make our choice, if it is done rationally, on the basis of such mental trial and error. The maximum control and foresight and the widest range of possible adjustment is in general attained by the individual who can make focal his motives in their skeletal and abstract form before they have absorbed any habitual or preconsciously preferred attributes. The

(122) thinker is the man who does not take for granted even that which is generally accepted as common sense. Even our habitual ways of doing things and our preconscious preferences are evaluated by him as though they were possible alternatives, but by no means as though they were the inevitable and only ones.

I have in this section shown the application of the various principles of intelligent adjustment that I have so far discussed. I have assumed a universal ideo-motor tendency for all conscious life. It is not sufficient to say that ideas tend sooner or later to express themselves in action. Ideas always tend to define themselves motorially. The effect of this universal tendency on the psychological object I have called particularization. Intelligent conduct implies the inhibition of a motive at an undefined stage in order to make it focal in its incomplete form. By so doing we are better able to select its further particulars and eliminate mentally lived failures from overt expression. This has also been described as essentially a process of trial and error which takes place mentally instead of overtly with consequent survival value for the organism. All conscious life is according to this point of view to be considered as unfinished action, and its biological value is to be judged in the light of the adaptive adjustments to which the conscious operations actually lead. Finally, an abstraction is an unfinished want or need which is focal for the purpose of selecting with foresight the means of neutralizing it. An abstraction of a high order is, accordingly, a want which is focal when it has only a relatively small number of attributes. To think is to test the tentative additional attributes by living their consequences imaginally.


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