Idiots are persons so deeply defective in mind from birth or from an early age that they are unable to guard themselves from common physical dangers, such as, in the case of young children, would prevent their parents from leaving them alone.
Imbeciles are persons who are capable of guarding themselves from common physical dangers, but who are incapable of earning their own living by reason of mental defects existing from birth or from an early age.
Feeble-minded are persons who may be capable of earning a living under favorable circumstances, but are incapable from mental defect existing from birth or from an early age: (a) of competing on equal terms with their normal fellows; or (b) of managing themselves and their affairs with ordinary prudence.
While it is possible to measure quite precisely the degree of defect in cases of arrest of development, it is hardly possible in the present state of our knowledge to define exactly the nature of it. Perhaps most clearness on the subject is to be gained from Binet's conception of intelligence, which emphasizes three characteristics of the thought process: (1) Its tendency to take and maintain a definite direction; (2) the capacity to make adaptations for the purpose of attaining a desired end; and (3) the power of autb-criticism.1
1 The "intelligence quotient " is the ratio of mental age to chronological age. It is used particularly in connection with the Stanford revision of the Binet-Simon intelligence scale.
2 Lewis M. Terman. The Measurement of Intelligence. Boston, 1916.
As an illustration may be taken one of Binet's series of tests, that of arranging five weights, which is normally passed at the age of nine years:
"Success depends, in the first place, upon the correct comprehension of the task and the setting of a goal to be attained; secondly, upon the choice of a suitable method for realizing the goal; and finally, upon the ability to keep the end clearly in consciousness until all the steps necessary for its attainment have been gone through. Elementary as are the processes involved, they represent the prototype of all purposeful behavior. The statesman, the lawyer, the teacher, the physician, the carpenter, all in their own way and with their own materials, are continually engaged in setting goals, choosing means, and inhibiting the multitudinous appeals of irrelevant and distracting ideas. In this experiment the subject may fail in any one of the three requirements of the test or in all of them. (1) He may not comprehend the instructions and so be unable to set the goal. (2) Though understanding what is expected of him, he may adopt an absurd method of carrying out the task. Or (3) he may lose sight of the end and begin to play with the blocks, stacking them on top of one another, building trains, tossing them about, etc.
"However, an examination of the scale will show that the choice of tests was not guided entirely by any single formula as to the nature of intelligence. Binet's approach was a many-sided one. The scale includes tests of time orientation, of three or four kinds of memory, of apperception, of language comprehension, of knowledge about common objects, of free association, of number mastery, of constructive imagination, and of ability to compare concepts, to see contradictions, to combine fragments into a unitary whole, to comprehend abstract terms, and to meet novel situations."2
1 Binet and Simon. Vintelligence des imbeciles. L'Annde Psy-chologique, 1919. - Lewis M. Terman. Loc. cit.
2 Lewis M. Terman. Loc. cit.
Usually both normal and defective subjects furnish more or less scattered results upon application of intelligence scales, i.e., they pass in some tests at higher age levels and fail in others at lower ones; and clinicians have often reported cases of arrest of development in which normal or even phenomenal mental capacity was observed in certain limited directions - memory, calculating ability, musical ability, etc. It would seem from all this that the nature of the defect is not the same in all cases of arrest of development, but is merely sufficiently pronounced and sufficiently general, in respect to the mental faculties involved, to seriously interfere with the patient's power of adjustment to ordinary conditions of life.
The defect of intelligence in cases of idiocy and imbecility is generally alone sufficient to produce serious social maladjustment. But in many cases of moronism and borderline conditions it is not the defect of intelligence, but some accompanying temperamental abnormality that is the main source of trouble. Many a moron of a mental age of, say, nine or ten years, being of pleasant disposition, industrious, and obedient, leads an uneventful existence as a useful member of the community, while many another, presenting no greater defect of intelligence, but being indolent or vicious, becomes a problem for the public authorities through dependency, vagrancy, prostitution, incendiarism, or other antisocial behavior.