The Nature of Intelligence

Chapter 5. The Principle of Protective Adjustment

L.L. Thurstone

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1. The receptors and bodily risk
2. Receptors and the control of futures
3. Abstraction as control of futures


The phylogenetic development of intelligence can be unified in what we may call the principle of protective adjustment. The application of intelligence in any form of adjustment is an effort to satisfy the wants of the organism with the least possible physical risk. This applies not only to the higher forms of intelligent conduct in man but it applies as well to animal intelligence.

An organism which has no differentiated sense-organs is compelled to expose its main body to the unknown conditions of the environment in satisfying its wants. Such an organism is not able to explore the environment except by coming in bodily contact with the environmental facts. This limitation is made still more serious if the organism possesses no differentiated contact receptors which can be sent out to explore the unknown situation before exposing the main body to it. It should be obvious that such an organism satisfies the wants of its normal life at considerable risk to its physical self.


Consider the advantage of the organism which is equipped with exploring members such as vibrissae or tentacles. These protect the main body trunk and can be used to explore an environmental situation before exposing the main body to it. Such differentiated receptors constitute landmarks in the phylogenetic development of intelligence. The development of sense-organs is continuous with the very highest forms of human intelligence. The sense-impressions from the exploring member have biological value only in so far as they are identified by the animal with the equivalent contact experience which would be encountered if the sense-impressions were ignored. To perceive, then, by means of an exploring member is to equate the sense-impression with the equivalent contact experience. If that expected contact experience fits with the momentary purpose, the adjustment issues into overt form. If the sense-impressions disagree with the momentary purpose the sense-impressions are either ignored or avoided by turning in some other direction for different stimuli. Even the lowest forms of differentiated contact receptors give the animal some control over future time and over distance. In these respects the efficiency afforded by the contact receptors is of the same variable as the efficiency which we claim by our human intelligence or foresight. The contact receptors have their biological justification in the increased facility for satisfying bodily wants with a minimum of bodily exposure and risk.

It is of course obvious that the distance receptors serve the same purpose as the contact receptors except that the control is extended still more into future time and distance.

(71) In this sense there is a psychological identity between future time and space. Smell is a distance-sense department which has its nearest correlate in the contact-sense department of taste. To perceive by smell is to live imaginally the expected taste experience. The smell percept is in fact the imaginal equivalent of the contact experience of taste. The contact experience is accordingly consummated, ignored, or avoided, depending on the momentary purposes.

This equivalence of the sense-impressions through the distance and contact receptors is obvious enough as far as smell and taste are concerned. The equivalence for vision or hearing and their corresponding contact experiences is not so often noted.

Consider at random any visual percept such as the percept of a knife. To perceive the knife is to feel ready to pick it up, to handle it, to use it. If the surface of the blade looks smooth, this visual smoothness and polish are perceived as expected smoothness to the touch and expected ease of cutting. To perceive through the distance receptors is to live imaginally the expected contact experience.

The term " meaning " covers the particularization of the sense-impression that I am here discussing. It is, however, not adequate to consider meaning as though it were simply an equation or symbolic representation. The meaning of the visual impressions from the knife is not simply a lot of details that are equated to the visual impression. It is not adequate to say that the sense-impression A " means " B, C, D, in the form of an equation. The sense-impression A completes itself by adding attributes which become an imaginal contact experience. It should be clear that meaning

( 72) does not refer merely to associated attributes. It refers to the ever-present ideo-motor tendency of sense-impressions to particularize themselves into expected experience. If these imaginal experiences fit our momentary purposes we live them out into overt form. If they do not fit our various motives at the moment we ignore or inhibit the execution of the meaning. If we interpret the term "meaning" in this way we bring it into line with a mind that works, and not merely with an abstract description of associated mental elements. The term " meaning " is in a sense synonymous with particularization with the exception that in its ordinary use it is restricted to a narrower range of definition of the psychological object than what I have tried to convey by the term "particularization".

I should like to call attention to the fact that Vierordt's law with reference to local sign can be related to the principle of protective adjustment. Vierordt's law states that the local sign discrimination is keenest in those skin surfaces which are farthest removed from the joints and body trunk and which are most movable. The biological justification for this type of discrimination is probably in the increased facility for exploring the environment. It is therefore consistent that we should find this type of discrimination keenest on the skin surfaces of the exploring members of the body. It is keener on the finger tips than on the forearm, keener there than on the shoulder, and keener there than on the back.

Consider yourself walking in the dark. You would perhaps use the foot as an exploring member before trusting your body weight on it. This protects the main body from the

( 73) unknown conditions into which you would step. If you have a cane handy it would be used as an exploring member by which the unknown conditions of the immediate future could be controlled with the least possible bodily risk. If a flashlight were available, it would give more facility in controlling the immediate future, and it would enable you to inhibit false steps without ever executing them. With daylight the expected contact experience can be roughly anticipated even hours before it is encountered. That is what visual perception actually consists of, namely the partial living through of the consummatory act with its consequences.


The contact and distance receptors show an increase of control over the environment in the expression of bodily wants. The next higher form in the development of intelligence is the capacity to imagine the desired stimulus before it is perceptually present even to the distance receptors. Suppose that you are driving along a country road and that you notice that your gasolene supply is getting low. The thought may become focal in a fleeting way that you will stop at the next gasolene station. When the next gasolene station becomes visible you may be thinking and talking about entirely different things, but since the motive to get some more gasolene has not yet been satisfied you are more than ordinarily sensitive to stimuli with such reference. The sign as a stimulus therefore becomes focal. Shortly after you have filled the gasolene tank you may pass other stations without even seeing the signs. Your sensitivity

(74) for that kind of stimulus is then low because there is not in you any unsatisfied motive which can express itself through the subsequent signs.

Suppose that you have on several occasions been vaguely dissatisfied with the disorderly appearance of your current magazines and catalogues which you now have in a heap on top of a table. They do not present the orderly appearance which would be consistent with that of your other belongings. You may never have stopped to consider any remedy. You may never even have made any comment about it. You are walking along the sidewalk with a friend discussing the events of the day. Your office is not focal and perhaps not even marginal. Nevertheless you stop at an office furniture window to look at a filing case which might be suitable for the orderly keeping of the magazines and catalogues. In this case an unsatisfied motive, the expression or solution of which has never been focal, constitutes the source of an unusual sensitivity to those stimuli which might serve to particularize and express that motive. You may never have thought of a special filing case for your magazines and catalogues, and yet your feeling of dissatisfaction about their appearance would make you sensitive to those stimuli which might constitute suitable particularizations of the motive or dissatisfaction.

These two illustrations show that the motive, which is simply an unsatisfied want, is primarily responsible for our sensitiveness to the stimuli of the environment. The motive may have been focal as an expressed want. This was the case in the gasolene illustration. The motive may have been focal simply as a dissatisfaction with reference to some

( 75) psychological object without being developed far enough to make focal any proposed expression or solution of it. That was the case with the dissatisfaction about the magazines. There is no sharp line of demarcation between these. They differ only in the extent to which the motive has been particularized when it was last focal. It is essential to keep in mind that the motive and the Percept are not two different things. The Percept is simply the more completely defined motive. The appearance of the stimulus can be interpreted as synonymous with the attitude, " Look, this is the particular way in which I might satisfy my want." The perceptual presence of the stimulus is a partial delimitation and expression of the motive. The interest or attention that we give to a stimulus depends first of all on its possibility as an avenue of expression for our desires.

Most of the stimuli that we encounter in our everyday life are actually looked for. The motive defines itself along some avenue and becomes focal as a psychological object which is then searched for by more or less random or guided moving about. When the stimulus is found, it consists of the searched-for psychological object and the perceptual detail around it serves further to define the overt expression of the motive. For example, you are about to leave your house and it is the time of the first cold weather of the winter. You decide to use your fur cap. Your motive has become particularized as the verbal imagery of the fur cap and you search for that stimulus. On your way to the wardrobe you expect to find the stairway, which serves as a stimulus to step on it; you expect to find the switch in the wardrobe

( 76) which serves as stimulus to turn on the light. Most of the stimuli that we encounter are expected either focally or marginally.

The capacity to expect a stimulus before it is present obviously has survival value. We are fortunate in this capacity because we can satisfy our wants not only in terms of the piece of environment with which we are actually now in contact, and through the neighbouring environment which we partly control through our distance receptors, but also through the imaginally expected stimuli into which our purposes have defined themselves and which we can set about systematically to look for.

Ideation is the process of putting detail into our motives. Ideation is in this respect the process of completing that which starts at the motive end of the reflex circuit. There is another aspect of ideation with reference to the terminal end of the circuit to which I should like also to call attention. Ideation is not only to be considered as the process of completing or defining the motive. Every idea has a retrospective reference to its source in a motive or dissatisfaction, and it has also a forward or anticipatory reference to that which we expect to experience. Ideation is in this sense as close as possible a statement of that which we should like to experience, or that which we expect to face.

There is not infrequently in the mind of the student of psychology a misunderstanding regarding the difference between perception and imagination. It is often thought that perception is solely the sense-impressions with the attributes to which they are equated, and that imagination is something different in that it does not start with or depend

( 77) on a perceptually present provocation. One is supposed to be externally caused, and the other is supposed to be internally caused. This is a cleavage between these two psychological categories which does not in reality exist. The misunderstanding is probably caused by the misinterpretation of what is to be included in the category of " meaning ". Practically all of the function of perception is imaginal. It is imaginal, to be sure, on a very completely defined stage. Perception may be treated as a form of imagination which is primarily kinesthetic in its reference and close to the motorium. The only part of the perception which is not imaginal is the sense-impression, but if the perceiving is at all intelligent the sense-impression elaborates readily into an imaginal contact experience with the perceived object. The essential Part of the percept is the imaginal experience with the Perceived object. The sense-impression is merely a cue defining the final detail of the contemplated act.

Consider the visual sense-impression from the sign " This way out " with an arrow-head indicating the direction. The sense-impression is here only a cue which helps to particularize the final muscular detail of my purpose to get out. The kinesthetic and visual imagery of completing this purpose is the essential part of the percept. The sense-impressions from the sign are only a relatively trivial part of the perceiving. The course of the imaginal anticipation of completing the purpose to get out is delimited by the sense impression. It is a shirt cut by which one avoids both mental and overt trial and error. The sensorial cue serves as one of the final delimiting agents in the definition of the purpose.

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It is of course possible to perceive without expecting either imaginally or by intention to complete any overt adjustment involving the perceived object. For example, you may perceive a small dot on a sheet of paper without having any intention to do anything about it. You may insist that you are perceiving in this case without entertaining any imaginal contact experience with the paper and the dot. You are correct. If you persist in perceiving the dot you will do so by adding attributes to the sensorial cue which were not present at its first appearance. You will think, perhaps, like this : " dot, round dot, pencil dot, it is so far away, it is in the middle of the sheet of paper, I see it clearly now, the light reflects on one side of the dot, I wonder if it punched through the paper," and so on. These additional attributes all look aimlessly toward completion of some sort. All this aimless perceiving is admittedly possible, but if in our daily life we should indulge in very much of this psychological laboratory perception, we should not long be allowed to take care of ourselves.

It is entirely possible for us to exercise our mental machinery on exhibit, as it were, without having any purpose in mind. One can do the same thing with a machine. You may punch the keys and levers of an adding machine just to show that you can operate the machine without having any need for it just now. That does not disprove that the machine has a normal purpose for which it was built. It is just so with mental life. WP may sit in a psychological laboratory and prove by universal consensus that we can have mental operations without any purpose or objective whatever. That does not prove that our normal images,

(79) ideas, and thoughts, are not expressions of our purposes and wants. It is even possible to look at the dot and to inhibit the natural ideo-motor tendency of the sense-impression to particularize itself by adding new attributes. That is called auto-hypnosis and the result is that focal consciousness disappears altogether.

The fundamental difference between imagination and perception is that imagination precedes perception in the reflex circuit. Let us limit ourselves for the moment to imagination of the ideational sort in which concretely specified situations are imagined. Perception is closer to the motorium, and imagination is closer to the universal end of the definition of the motive. The development of intelligence is measurable by the incompleteness of the motive at which it can become focal. If the motive becomes focal as an imaginal experience before the stimulus has even appeared, the animal has in consequence of this capacity an increased control in the satisfaction of its wants with minimum risk. To imagine is to anticipate mentally the detailed experience by which the motive would be neutralized. Such a state of mind lowers the threshold for relevant stimuli and these can then be searched for with greater flexibility of adjustment than if the animal were compelled to wander about at random in the hope of stumbling over a promising stimulus. Imagination can be thought of as the capacity to define a motive to the stage of a concrete experience by which the motive would he satisfied, and the capacity to do this 'without the assistance of the stimulus. In a less mature form of intelligence the motive would express itself to the point of vague dissatisfaction issuing into random overt

( 80) running about. This level of intelligence would be capable of particularizing the motive in terms of the present stimulus when found, but it would not be capable of particularizing the motive to the point of a concrete experience in the absence of the stimulus. The process is essentially the same in kind. The difference is only in the degree of incompleteness of the motive at which it becomes focal, and the capacity to particularize the motive without the assistance of the perceptually present particulars. Imagination is identical with perception with the sensorial cue omitted. The mind that we are here discussing is progressing to higher and higher intelligence in that the motive becomes focal at earlier and earlier stages in the circuit when it contains fewer and fewer attributes. At the highest levels of intelligence we shall have the motive becoming focal when it contains only those attributes which define it as a universal.


Let us consider conceptual thinking as a form of protective adjustment. You are going to send a parcel to someone in another city. Will you send it express, or by the ordinary mail, or by parcel post ? When you think of your purpose " to send the package " you are in reality entertaining a purpose which is still unformed. If there is absolutely no conflict of impulses regarding the details of the act of sending the parcel you will not even stop to think about it. But as the specific acts by which you are about to send the package are just about to be completed, the unformed action is mental in its conceptual terms. The incomplete

(81) act " to send the package " defines itself successively in the three ways of sending it, and these are themselves concepts of lower order in that they are more defined, they contain more attributes than the arrested action at which they started. To think, even in so simple a situation as this, means to strip the final action as far as possible of the details which are not absolutely essential to identify it, and to be ready to repeat the process if the fortuitous particularization is not entirely satisfactory. Let us suppose that you decide to send the package by express. The decision to do so involves the partial definition of a concept which is still a concept of a lower order. Even at that moment your mental state does not perhaps contain in it the elements to determine whether the package is to be sent now or to-morrow morning, whether you will take it to the express office or telephone, and so on. As the action takes on sufficient details to involve your own bodily situation with reference to the expected experience, the mental state becomes an idea in the sense in which we have here defined it. The stimuli from the environment such as the sight of the telephone, the office, the appropriate label for the chosen manner of sending the package, these stimuli are expected and when found they help further to define the expected experience into reality.

Finally, let us consider the relationship between what has been called the recession of the stimulus and the principle of protective adjustment. The recession of the stimulus refers to the fact that with the advance of intelligence through the distance receptors and the capacity to imagine, we note effective adjustments being made in terms of more

( 82) and more inconspicuous stimuli. The lowest animal forms must be in physical contact with their food before random search can be abandoned in favour of the guided precurrent or consummatory adjustments. The presence of smell organs enables the organism to abandon random search sooner, and to instal guided precurrent adjustments earlier in its search for the stimulus. The distance receptors of vision and hearing allow random search to be abandoned still earlier in favour of precurrent adjustments which are guided by the foresight of the distance receptors. This progress involves a diminution in the size of the sense-impression with reference to the receptor surface, and a decrease in the physical intensity of the stimulus itself. The same progress can also be thought of as a withdrawal of the animal body from the stimulus in time and space, allowing more and more opportunity for protective precurrent adjustment. This is synonymous with a reduction in the element of chance in the discovery of the stimulus. With the higher development of perceptual intelligence, and with rudimentary imaginative control, we have still more inconspicuous stimuli regarded by the animal as significant for guided precurrent adjustment. This is because the relatively insignificant features of the environment come to have what w e know as meaning These insignificant features define themselves into expected experience. We credit intelligence as keen, when we find it capable of inferring the rest of a situation from the trivial perceptual marks of it. It is not a far step to eliminate the insignificant stimulus altogether in defining our purposes to the point of imaginal

(83) concrete experience. The recession of the stimulus is but another way of stating that increased intelligence means the increased capacity to express our wants with more and more independence of conspicuous and adjacent stimuli.

In these illustrations I have shown that the development of intelligence can be thought of as continuous, beginning in the differentiation of structure of lower animal forms by which a portion of the body is set aside for the risky job of exploring the environment, protecting the body as far as possible. The development which is marked at first by material and objective mechanical differentiation of function leads in the mental development to the highest forms of intelligence in man, and it can be thought of as continuous in this sense that it is all concerned with the control of distance and its psychological equivalent, future time. The differentiated contact receptors serve as an immediate form of protection. The distance receptors extend considerably the same type of protection in that they enable the organism to ward off useless action before it even becomes incipient. As long as the animal is limited to contact receptors, the accepted as well as the rejected action must be more or less imminent or incipient before its value can be judged. With the aid of distance receptors the control is considerably extended into future time and space.

The major portion of perceptual activity is imaginal in that the expected experience for which the distance-impression serves as a cue is only very remotely contained in the distance-impression itself. Perception is largely imaginal in its nature. The distance-impression itself on the receptor surface is only a very slight and relatively insignificant

( 84) portion of the experience of perceiving, compared with the imaginal or interpretative parts of it. It is therefore not a far step, after all, to realize that perceiving activity might well go on in its imaginal parts in the absence of the slight receptor stimulation. In this case we have imagination which is psychologically almost identical with perception except that it is less defined, less distinct, and more flexible to momentary alteration.

Let us now imagine the unfinished action which constitutes first perception, then, when still less defined, imagination, becoming less and less defined so that choices are made while the conflicting lines of conduct are still in their rough, general, loose, skeletal form, and we have conceptual thinking. The less there is of the impulse at the time when it is subject to rational acceptance or rejection, the higher is the mentality of the actor. Genius is essentially the capacity to deal effectively with impulses at the stage of formation when they are still only roughly defined affective states, before they have absorbed enough attributes to become the cognitive terms with which most of us are limited in our field of rational control. By the principle of protective adjustment I mean this unifying idea under which we can cover consistently the development of intelligence from its lowest forms to its highest, the development which gains for the organism increasing range of control of future action and consequently increasing security for the bodily self, a development which renders possible the choices of conduct which will find their overt execution in a more and more distant and removed future.


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