Chapter 22
WHAT PSYCHOLOGY IS AND DOES
THE SUBJECT-MATTER OF THE SCIENCE, ITS PROBLEMS AND ITS METHODS
Modern psychology is an attempt to bring the methods of scientific investigation, which have proved immensely fruitful in other fields, to bear upon mental life and its problems. The human individual, the main object of study, is so complex an object, that for a long time it seemed doubtful whether there ever could be real science here; but a beginning was made in the nineteenth century, following the lead of biology and physiology, and the work of the investigator has been so successful that to-day there is quite a respectable body of knowledge to assemble under the title of scientific psychology.
Psychology, then, is a science. It is the science of--what shall we say? "The science of the soul"--that is what the name means by derivation and ancient usage. "The science of the mind" has a more modern sound. "The science of consciousness" is more modern still. "The science of behavior" is the most recent attempt at a concise formula.
None of these formulas is wholly satisfactory. Psychology does not like to call itself the science of the soul, for that has a theological tang and suggests problems that have so far not seemed accessible to scientific investigation. Psychology does not like very well to call itself the science {2} of the mind, as _the_ mind seems to imply some thing or machine, and there is no such thing to be observed (unless it be the brain and body generally), and, anyway, psychology is distinctly a study of actions rather than of things. Psychology does not like to limit itself to the study of consciousness, but finds it necessary to study also unconscious actions. As to "behavior", it would be a very suitable term, if only it had not become so closely identified with the "behavioristic movement" in psychology, which urges that consciousness should be entirely left out of psychology, or at least disregarded. "Behavior psychology", as the term would be understood to-day, means a part of the subject and not the whole.
[Footnote: A series of waggish critics has evolved the following: "First psychology lost its soul, then it lost its mind, then it lost consciousness; it still has behavior, of a kind."]
The best way of getting a true picture of psychology, and of reaching an adequate definition of its subject-matter, would be to inspect the actual work of psychologists, so as to see what kind of knowledge they are seeking. Such a survey would reveal quite a variety of problems under process of investigation, some of them practical problems, others not directly practical.
Varieties of Psychology
Differential psychology.
One line of question that always interests the beginner in psychology is as to how people differ--how different people act under the same circumstances--and why; and if we watch the professional psychologist, we often find him working at just this problem. He tests a great number of individuals to see how they differ, and tries to discover on what factors their differences depend, how far on heredity, how far on environment. The "psychologist" in such a place as the children's court {3} is a specialist whose duty it is to test the delinquent children that are brought before the court, with the special object of measuring the intelligence of each individual child and of helping in other ways to understand the child's peculiar conduct and attitude.
The "psychological examiner" in the Army, during the Great War, had the same general object in view. It was desirable to measure the intelligence of each recruit as he entered the service, since military experience had shown that men of low intelligence made poor soldiers, while those of high intelligence made the best officers and non-commissioned officers, provided they also possessed good physique and certain less measureable mental qualifications, such as courage and leadership.
Applied psychology.
The Army psychologists, like the court psychologist, were engaged in applying scientific knowledge to the practical problems of life; and there are many other applications of psychology, to education, to medicine, to business and other occupations, as well as to the art of right living. Scientific knowledge enables you to _predict_ and _control_. Having devised scientific tests for intelligence, you can predict of a six-year-old boy who tests low, that he will not get much good from the regular classes in school; and thus you are in a position to control the education of this boy for his own best interests. In the Army, it happened during the earlier part of the war that some companies or regiments made much slower progress in training than others; and a whole Division was delayed for months because of the backwardness of a single regiment. When the psychological tests were introduced, these slow-learning units were found to contain a disproportionate number of men of low intelligence. From that time on, it was possible by aid of the tests to equalize the intelligence of different units when first formed, and thus insure equal {4} progress in training. This was a good example of "control".
Most of us are attracted by the practical use of a science, and some have no patience with any study that does not seem immediately practical. But really any science, however much it is applied, must remain fundamentally a pure science; that is, it must seek most of all to know and understand. Practical scientific knowledge was usually first obtained without any inkling of how it might be used. The science of electricity is the most striking example of this. It began as an attempt to understand certain curious phenomena, which seemed to be nothing but curiosities; yet when the knowledge of these phenomena had progressed to a certain point, abundant use was found for it. Much the same is true of psychology, which began as a pure science and only recently has found ways of applying its discoveries to practical affairs. So the student beginning the science, though properly desirous of making practical use of what he learns, should let himself be governed for the present by the desire to know and understand, confident that the more scientific (which is to say, the more complete, systematic and reliable) his knowledge is, the more available it will be for practical application.
General psychology.
Our science is not concerned entirely with differences between people, but asks also in what ways people are alike, and this is indeed its central problem. How do "we" observe, learn, remember, imagine, think? What sensations and feelings do we have, what emotions, what instincts, what natural and acquired impulses to action? How are our natural powers and impulses developed and organized as we grow up? Psychology is concerned with the child as well as the adult, and it is even concerned with the animal. It is concerned with the abnormal as well as the normal human being. So you will find books and {5} courses on animal psychology, child psychology, abnormal psychology. Now general psychology--or just plain "psychology"--has to do with the main laws and principles that hold in all these special fields.
Psychology as Related to Other Sciences
A good definition of our science would distinguish it from other sciences, especially from those neighboring sciences with which it is in closest contact.
Psychology and sociology.
There is no difficulty in framing a good logical distinction here. Sociology studies the activities of a group of people taken as a whole, while psychology studies the activities of the individuals. Both might be interested in the same social act, such as an election, but sociology would consider this event as a unit, whereas psychology would break it up into the acts of the several voters. The distinction is clear enough theoretically, but breaks down often in practice, as sociology would like to know the motives that swayed individual voters, while psychology on its side is interested to know what decision was reached by the majority. All the social sciences, including economics and politics, have a psychological side, since they evidently are concerned to know the causes that govern human conduct. Social psychology studies the individual in his social relations.
Psychology and biology.
Biology, being the science of living creatures, includes psychology, which studies these creatures on the mental side. The science of life includes the science of mental life. We may call psychology a part of biology, or we may call it one of the biological sciences. It has very close contact with several other branches of biology. Animal psychology overlaps that part of zoology which studies the behavior of animals. Genetic psychology, as it is sometimes called, i.e., the study of mental heredity. {6} and development, dovetails with the general biological science of genetics, so that we find biologists gathering data on the heredity of feeble-mindedness or of musical ability, while psychologists discuss the general theory of heredity.
Psychology and physiology.
That one of all the sciences that has the closest contacts with psychology is human and animal physiology. Broadly defined, physiology is that part of biology that studies functions or activities; and, so defined, it includes psychology as part of itself. In practice, psychology devotes itself to desire, thought, memory, and such "mental functions", while physiology concentrates its effort upon "bodily functions" like digestion and circulation. But this is only a rough distinction, which breaks down at many points.
Where shall we class sensation? Is it "mental" or "bodily"? Both sciences study it. Physiology is perhaps more apt to go into the detailed study of the action of the sense organs, and psychology to concern itself with the classification of sensations and the use made of them for recognizing objects or for esthetic purposes. But the line between the two sciences is far from sharp at this point.
Speech, also, lies in both provinces. Physiology has studied the action of the vocal organs and the location of the brain centers concerned in speech, while psychology has studied the child's process of learning to speak and the relation of speech to thought, and is more apt to be interested in stuttering, slips of the tongue, and other speech disturbances which are said to be "mental rather than physical".
It would be hard to mention any activity that is mental without being physical at the same time. Even thinking, which seems as purely mental as any, requires brain action; and the brain is just as truly a bodily organ as the heart or stomach. Its activity is bodily activity and lies properly within the field of physiology.
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But it would be equally difficult to mention any function that is exclusively bodily, and not mental at the same time, in some degree. Take digestion for example: the pleasant anticipation of food will start the digestive juices flowing, before any food is physically in the stomach; while in anger or fear digestion comes to a sudden halt. Therefore we find physiologists interested in these emotions, and psychologists interested in digestion.
We do not find any clean separation between our science and physiology; but we find, on the whole, that psychology examines what are called "mental" activities, and that it studies them as the performances of the whole individual rather than as executed by the several organs.
The Science of Consciousness
Typically, the activities that psychology studies are conscious performances, while many of those falling to physiology are unconscious. Thus digestion is mostly unconscious, the heart beat is unconscious except when disturbed, the action of the liver is entirely unconscious. Why not say, then, that psychology is the study of conscious activities?
There might be some objection to this definition from the side of physiology, which studies certain conscious activities itself--speech, for example, and especially sensation.
There would be objection also from the side of psychology, which does not wish to limit itself to conscious action. Take the case of any act that can at first be done only with close attention, but that becomes easy and automatic after practice; at first it is conscious, later unconscious, but psychology would certainly need to follow it from the initial to the final stage, in order to make a complete study of the practice effect. And then there is the "unconscious", or the "subconscious mind"--a matter on which psychologists {8} do not wholly agree among themselves; but all would agree that the problem of the unconscious was appropriate to psychology.
For all the objections, it remains true that the _typical_ mental process, the typical matter for psychological study, is conscious. "Unconscious mental processes" are distinguished from the unconscious activity of such organs as the liver by being somehow _like_ the conscious mental processes.
It would be correct, then, to limit psychology to the study of conscious activities and of activities akin to these.
The Science of Behavior
No one has objected so strenuously to defining psychology as the science of consciousness, and limiting it to consciousness, as the group of animal psychologists. By energetic work, they had proved that the animal was a very good subject for psychological study, and had discovered much that was important regarding instinct and learning in animals. But from the nature of the case, they could not observe the consciousness of animals; they could only observe their behavior, that is to say, the motor (and in some cases glandular) activities of the animals under known conditions. When then the animal psychologists were warned by the mighty ones in the science that they must interpret their results in terms of consciousness or not call themselves psychologists any longer, they rebelled; and some of the best fighters among them took the offensive, by insisting that human psychology, no less than animal, was properly a study of behavior, and that it had been a great mistake ever to define it as the science of consciousness.
It is a natural assumption that animals are conscious, but after all you cannot directly observe their consciousness, and you cannot logically confute those philosophers {9} who have contended that the animal was an unconscious automaton. Still less can you be sure in detail what is the animal's sensation or state of mind at any time; to get at that, you would need a trustworthy report from the animal himself. Each individual must observe his own consciousness; no one can do it from outside. The objection of the behaviorist to "consciousness psychology" arises partly from distrust of this method of inner observation, even on the part of a human observer.
Indeed, we can hardly define psychology without considering its _methods of observation_, since evidently the method of observation limits the facts observed and so determines the character of the science. Psychology has two methods of observation.
When a person performs any act, there are, or may be, two sorts of facts to be observed, the "objective" and the "subjective". The objective facts consist of movements of the person's body or of any part of it, secretions of his glands (as flow of saliva or sweat), and external results produced by these bodily actions--results such as objects moved, path and distance traversed, hits on a target, marks made on paper, columns of figures added, vocal or other sounds produced, etc., etc. Such objective facts can be observed by another person.
The subjective facts can be observed only by the person performing the act. While another person can observe, better indeed than he can himself, the motion of his legs in walking, he alone can observe the sensations in the joints and muscles produced by the leg movement. No one else can observe his pleased or displeased state of mind, nor whether he is thinking of his walking or of something quite different. To be sure, his facial expression, which is an objective fact, may give some clue to his thoughts and feelings, but "there's no art to read the mind's construction {10} in the face", or at least no sure art. One may feign sleep or absorption while really attending to what is going on around. A child may wear an angelic expression while meditating mischief. To get the subjective facts, we shall have to enlist the person himself as our observer.
Introspection
This is observation by an individual of his own conscious action. It is also called subjective observation. Notice that it is a form of observation, and not speculation or reasoning from probabilities or from past experience. It is a direct observation of fact.
One very simple instance of introspection is afforded by the study of after-images. Look for an instant at the glowing electric bulb, and then turn your eyes upon a dark background, and observe whether the glowing filament appears there; this would be the "positive after-image". This simple type of introspection is used by physiology in its study of the senses, as well as by psychology; and it gives such precise and regular results that only the most confirmed behaviorists refuse to admit it as a good method of observation.
But psychology would like to make introspective observations on the more complex mental processes as well; and it must be admitted that here introspection becomes difficult. You cannot hope to make minute observations on any process that lasts over a very few seconds, for you must let the process run its natural course unimpeded by your efforts at observing it, and then turn your "mental eye" instantly back to observe it _retrospectively_ before it disappears. As a matter of fact, a sensation or feeling or idea hangs on in consciousness for a few seconds, and can be observed in this retrospective way. There is no theoretical objection to this style of introspection, but it is practically difficult and {11} tricky. Try it on a column of figures: first add the column as usual, then immediately turn back and review exactly what went through your mind in the process of adding---what numbers you spoke internally, etc. Try again by introspecting the process of filling in the blanks in the sentence:
"Botany could not make use of introspection because ______ have probably no ________ processes."
At first, you may find it difficult to observe yourself in this way; for the natural tendency, when you are aiming at a certain result, is to reach the goal and then shift to something else, rather than to turn back and review the steps by which you reached the goal. But with practice, you acquire some skill in introspection.
One difficulty with introspection of the more complex mental processes is that individuals vary more here than in the simpler processes, so that different observers, observing each his own processes, will not report the same facts, and one observer cannot serve as a check upon another so easily as in the simpler introspection of after-images and other sensations, or as in the observations made in other sciences. Even well trained introspectionists are quite at variance when they attempt a minute description of the thought processes, and it is probable that this is asking too much of introspection. We mustn't expect it to give microscopic details. Rough observations, however, it gives with considerable certainty. Who can doubt, for example, that a well-practised act goes on with very little consciousness, or that inner, silent speech often accompanies thinking? And yet we have only introspection to vouch for these facts.
Objective Observation
But to say, as used to be said, that psychology is purely an introspective science, making use of no other sort of observation, is absurd in the face of the facts.
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We have animal psychology, where the observation is exclusively objective. In objective observation, the observer watches something else, and not himself. In animal psychology, the psychologist, as observer, watches the animal.
The same is true of child psychology, at least for the first years of childhood. You could not depend on the introspections of a baby, but you can learn much by watching his behavior. Abnormal persons, also, are not often reliable introspectionists, and the study of abnormal psychology is mostly carried on by objective methods.
Now how is it with the normal adult human being, the standard subject for psychology? Does he make all the observations on himself or may he be objectively observed by the psychologist? The latter, certainly. In fact, nearly all tests, such as those used in studying differential psychology, are objective. That is to say that the person tested is given a task to perform, and his performance is observed in one way or another by the examiner. The examiner may observe the _time_ occupied by the subject to complete the task, or the _quantity_ accomplished in a fixed time; or he may measure the correctness and _excellence_ of the work done, or the _difficulty_ of the task assigned. One test uses one of these measures, and another uses another; but they are all objective measures, not depending at all on the introspection of the subject.
What is true of tests in differential psychology is true of the majority of experiments in general psychology: the performer is one person, the observer another, and the observation is objective in character. Suppose, for example, you are investigating a memory problem; your method may be to set your subject a lesson to memorize under certain defined conditions, and see how quickly and well he learns it; then you give him another, equally difficult lesson to be learned under altered conditions, and observe whether he {13} does better or worse than before. Thus you discover which set of conditions is more favorable for memorizing, and thence can infer something of the way in which memorizing is accomplished. In the whole experiment you need not have called on your subject for any introspections; and this is a type of many experiments in which the subject accomplishes a certain task under known conditions, and his success is objectively observed and measured.
There is another type of objective psychological observation, directed not towards the success with which a task is accomplished, but towards the changes in breathing, heart beat, stomach movements, brain circulation, or involuntary movements of the hands, eyes, etc., which occur during the course of various mental processes, as in reading, in emotion, in dreaming or waking from sleep.
Now it is not true as a matter of history that either of these types of objective observation was introduced into psychology by those who call themselves behaviorists. Not at all; experiments of both sorts have been common in psychology since it began to be an experimental science. The first type, the success-measuring experiment, has been much more used than introspection all along. What the behaviorists have accomplished is the definitive overthrow of the doctrine, once strongly insisted on by the "consciousness psychologists", that introspection is the only real method of observation in psychology; and this is no mean achievement. But we should be going too far if we followed the behaviorists to the extent of seeking to exclude introspection altogether, and on principle. There is no sense in such negative principles. Let us accumulate psychological facts by any method that will give the facts.
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General Laws of Psychological Investigation.
Either introspective or objective observation can be employed in the _experimental attack_ on a problem, which consists, as just illustrated in the case of memory, in controlling the conditions under which a mental performance occurs, varying the conditions systematically, and noting the resulting change in the subject's mental process or its outcome. Psychologists are inclined to regard this as the best line of attack, whenever the mental activity to be studied can be effectively subjected to control. Unfortunately, emotion and reasoning are not easily brought under control, and for this reason psychology has made slower progress in understanding them than it has made in the fields of sensation and memory, where good experimental procedure has been developed.
Another general line of attack worthy to be mentioned alongside of the experimental is the _comparative method_. You compare the actions of individuals, classes or species, noting likenesses and differences. You see what behavior is typical and what exceptional. You establish norms and averages, and notice how closely people cluster about the norm and how far individuals differ from it. You introduce tests of various sorts, by which to get a more precise measure of the individual's performance. Further, by the use of what may be called double comparison, or "correlation", you work out the relationships of various mental (and physical) traits. For example, when many different species of animals are compared in intelligence and also in brain weight, the two are found to correspond fairly well, the more intelligent species having on the whole the heavier brains; from which we fairly conclude that the size of the brain has something to do with intelligence. But when we correlate brain weight and intelligence in human individuals. {15} we find so many exceptions to the rule (stupid men with large brains and gifted men with brains of only moderate size) that we are forced to recognize the importance of other factors, such as the perfection of the microscopic structure of the brain.
Tests and correlations have become so prominent in recent psychological investigation that this form of the comparative method ranks on a par with the strict experimental method. A test is an experiment, in a way, and at least is often based upon an experiment; but the difference between the two lines of attack is that an experiment typically takes a few subjects into the laboratory and observes how their mental performances change with planfully changed conditions; whereas a test goes out and examines a large number of persons under one fixed set of conditions. An experiment belongs under what we called "general psychology", and a test under "differential psychology", since the first outcome of a test is to show how the individual differs from others in a certain respect. The results may, however, be utilized in various ways, either for such practical purposes as guiding the individual's choice of an occupation, or for primarily scientific purposes, such as examining whether intelligence goes with brain size, whether twins resemble each other as much mentally as they do physically, whether intellectual ability and moral goodness tend on the whole to go together, or not.
The _genetic method_ is another of the general lines of attack on psychological problems. The object here is to trace the mental development of the individual, or of the race. It may be to trace the development either of mentality in general, or of some particular mental performance. It may be to trace the child's progress in learning to speak, or to follow the development of language in the human species, from the most primitive tongues up to those of the great {16} civilized peoples of to-day. It may be to trace the improvement of a performance with continued practice.
The value of the genetic method is easily seen. Usually the beginnings of a function or performance are comparatively simple and easy to observe and analyze. Also, the process of mental growth is an important matter to study on its own account.
The _pathological method_ is akin to the genetic, but traces the decay or demoralization of mental life instead of its growth. It traces the gradual decline of mental power with advancing age, the losses due to brain disease, and the maladaptations that appear in insanity and other disturbances. Here psychology makes close contact with _psychiatry_ which is the branch of medicine concerned with the insane, etc., and which in fact has contributed most of the psychological information derived from the pathological method.
The object of the pathological method is, on the one side, to understand abnormal forms of mental life, with the practical object of preventing or curing them, and on the other side, to understand normal mental life the better. Just as the development of a performance throws light on the perfected act, so the decay or disturbance of a function often reveals its inner workings; for we all know that it is when a machine gets out of order that one begins to see how it ought to work. Failure sheds light on the conditions of success, maladaptation throws into relief the mental work that has to be done by the normal individual in order to secure and maintain his good adaptation. According to the psychiatrists, mental disturbance is primarily an affair of emotion and desire rather than of intellect; and consequently they believe that the pathological method is of special importance in the study of the emotional life.
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Summary and Attempt at a Definition
Having now made a rapid preliminary survey of the field of psychology, and of the aims and methods of the workers in this field, we ought to be in a position to give some sort of a definition.
We conclude, then: psychology is a part of the scientific study of life, being the science of mental life. Life consisting in process or action, psychology is the scientific study of mental processes or activities. A mental activity is typically, though not universally, conscious; and we can roughly designate as mental those activities of a living creature that are either conscious themselves or closely akin to those that are conscious. Further, any mental activity can also be regarded as a physiological activity, in which case it is analyzed into the action of bodily organs, whereas as "mental" it simply comes from the organism or individual as a whole. Psychology, in a word, is the science of the conscious and near-conscious activities of living individuals.
Psychology is not interested either in dead bodies or in disembodied spirits, but in living and acting individuals.
One word more, on the _psychological point of view_. In everyday life we study our acquaintances and their actions from a personal standpoint. That is, we evaluate their behavior according as it affects ourselves, or, perhaps, according as it squares or not with our standards of right and wrong. We always find something to praise or blame. Now, the psychologist has no concern with praise and blame, but is a seeker after the facts. He would know and understand human actions, rather than pass judgment on them. When, for example, he is introduced into the school or children's court, for the purpose of examining children that are "problems", his attitude differs considerably from that of the {18} teacher or officer of the law; for while they almost inevitably pass judgment on the child in the way of praise or blame, the psychologist simply tries to understand the child. The young delinquent brought into the laboratory of the court psychologist quickly senses the unwonted atmosphere, where he is neither scolded nor exhorted, but asked to lend his coöperation in an effort to discover the cause why his conduct is as it is. Now, this psychological attitude is not necessarily "better" than the other, but it is distinctly valuable in its place, as seen from the fact that the young delinquent often does coöperate. He feels that if the psychologist can find out what is the trouble with him, this may help. Nothing, indeed, is more probable; it is when we have the facts and trace out cause and effect that we are in a fair way to do good. Nothing is more humane than psychology, in the long run, even though the psychologist may seem unfeeling in the course of his investigation.
To the psychologist, conduct is a matter of cause and effect, of natural law. His business is to know the laws of that part of nature which we call human nature, and to use these laws, as fast as discovered, for solving the problems presented by the human individual or group. For him, even the most capricious conduct has its causes, even the most inexplicable has its explanation--if only the cause can be unearthed, which he does not pretend he can always actually accomplish, since causes in the mental realm are often very complex. No one can be a psychologist all of the time; no one can or should always maintain this matter-of-fact attitude towards self and neighbor. But some experience with the psychological attitude is of practical value to any one, in giving clearer insight, more toleration, better control, and even saner standards of living.
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EXERCISES
1. Outline the chapter. A sample outline of the briefer sort is here given:
A. Subject-matter of psychology: mental activities.
(1) A sub-class under vital activities.
(2) Activities of individuals, as distinguished from
(a) Activities of social groups (sociology).
(b) Activities of single organs (physiology).
(3) Either conscious, or closely related to conscious activities.
(4) May be activities of human or animal, adult or child, normal or abnormal individuals.
B. Problems of psychology:
(1) How individuals differ in their mental activities.
(2) How individuals are alike in their mental activities.
(3) Practical applications of either (1) or (2).
C. Methods of psychology:
(1) Methods of observing mental activities.
(a) Introspective, the observing by an individual of his own actions.
(b) Objective, the observation of the behavior of other individuals.
(2) General lines of attack upon psychological problems.
(a) Experimental: vary the conditions and see how the mental activity changes.
(b) Comparative: test different individuals or classes and see how mental activity differs, etc.
(c) Genetic: trace mental development.
(d) Pathological: examine mental decay or disturbance.
2. Formulate a psychological question regarding each of the following: hours of work, genius, crime, baseball.
3. Distinguish introspection from theorizing.
4. What different sorts of objective fact can be observed in psychology?
5. What is the difference between the physiology of hearing and the psychology of hearing?
6. State two reasons why it would be undesirable to limit psychology to the introspective study of consciousness.
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7. What is the difference between an experiment and a test, (a) in purpose, (b) in method?
8. Compare the time it takes you to add twenty one-place numbers, arranged in a vertical column, and arranged in a horizontal line, (a) Is this introspective or objective observation? Why so? (b) Is it a test or an experiment? Why?
9. Write a psychological sketch of some one you know well, taking care to avoid praise and blame, and to stick to the psychological point of view.
REFERENCES
Some of the good books on the different branches of psychology are the following:
On animal psychology:
Margaret F. Washburn, _The Animal Mind_, 2nd edition, 1917.
John B. Watson, _Behavior_, 1914.
On child psychology:
Norsworthy and Whitley, _The Psychology of Childhood_, 1918.
On abnormal psychology:
A. J. Rosanoff, _Manual of Psychiatry_, 5th edition, 1920.
On applied psychology:
Hollingworth and Poffenberger, _Applied Psychology_, 1917.
On individual psychology, parts of:
E. L. Thorndike, _Educational Psychology, Briefer Course_, 1914,
Daniel Starch, _Educational Psychology_, 1919.
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