Whatever form the definition of feeble-mindedness may take, in this country at least[3] the concept has become quite firmly established as describing the condition of those who require social guardianship, because, with training, they do not develop enough mentally to live an independent life in society. The feeble-minded are socially deficient because of a failure to develop mentally. They are proper wards of the state because of this mental deficiency. Goddard says, they are “incapable of functioning properly in our highly organized society” (_112_, p. 6). The most generally quoted verbal description of the upper line of social unfitness is that of the British Royal Commission on Feeble-Mindedness: “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.” It is clear that the intention is to distinguish mental deficiency from senile dementia, from hysteria and from insanity, in which there is a temporary or permanent loss of mental ability rather than a failure to develop. Feeble-mindedness may, however, arise from epilepsy or from other diseases or accidents in early life as well as from an inherent incapacity for development. Moreover, _mental_ deficiency, or feeble-mindedness, (I use the terms interchangeably) does not imply that the social unfitness is always caused by intellectual deficiency. Mind is a broader term than intellect, as we shall note in the next section. This definition of the feeble-minded is the main idea expressed by Witmer (_221_), Tredgold (_204_), Pearson (_164_), and Murdock (_164_). The historical development of the concept is traced by Rogers (_172_) and Norsworthy (_159_). It is criticized by Kuhlmann (_140_) as impractical and indefinite. The indefiniteness is indicated by such terms as “under favorable circumstances,” “on equal terms,” and “with ordinary prudence.” This objectionable uncertainty as to social fitness can be considerably relieved for those types of feeble-mindedness which involve the inability to pass mental tests, since this result can later be correlated with subsequent social failure and predictions made during childhood on the basis of the tests. Attempts to make the concept of feeble-mindedness more definite have, therefore, naturally taken some quantitative form in relation to objective tests. Binet and the French commission in 1907 (_77_) called attention to the method in use in Belgium for predicting unfitness objectively on the basis of the amount of retardation in school at different ages. With the appearance in 1908 of the Binet-Simon revised scale for measuring mental development, quantitative descriptions began to be concerned with the borderlines of mental deficiency on scales of tests.