It has been stated that most of the mental functions, which human beings perform, are not elementary, but are capable of analysis into many contributing factors. Reading has been shown by such analysis to be a very complex function, interference in any part of which may result in disability. The causes of failure to learn to read under instruction, therefore, differ from child to child. Huey, who spent years studying the psychology of reading, finally became so imbued with the wonder of the process, that he felt that to know it in all its aspects and ramifications would be to know all psychology. Correlations between IQ and reading ability, among children of the same age, in both silent and oral reading, are positive and very high. This is especially true of reading for the understanding of sentences. Correlation between general intelligence, as measured by a scale like Stanford-Binet, and reading ability, as measured by a scale like Trabue’s Language Completion, or Thorndike-McCall’s scale for understanding of sentences, reaches as high as .90, and hardly ever in any group falls below .60. These correlations indicate that general mental maturity is very closely related to learning to read. The very intelligent children are the best readers in by far the majority of cases, while school children who do not learn to read under ordinary instruction, are usually feeble-minded. On the basis of experimentation in this field, Ranschburg suggests that even so mechanical an aspect of reading as ability to call correctly words exposed in a tachistoscope, may serve as a rough means of separating feeble-minded school children from the others. Nevertheless, even with correlation coefficients reaching as high as .90, there may occur occasional cases of very marked discrepancy between general intelligence and ability to read. Very early reading, with little or no formal instruction, is often found among children of very high IQ. Of four children measuring over 180 IQ (Stanford-Binet), found by the present writer in New York City, every one learned to read simple matter fluently during or before the third year of life. Their early mastery of reading was but a symptom of their great general capacity. Just what degree of intellectual development is typically reached before children can be taught to read is not known, but it is probably not far from a 6-year level. That is, children of ordinary intelligence can learn to read after they have passed their sixth birthday. A child who can read fluently at a mental age much below this must be considered to show a special ability; while one who cannot begin to learn at or above this general level[10] is afflicted with a special defect, in some of the functions which enter into the reading process. These functions may be classified as those which enter into _mechanics_, and those which enter into _comprehension_, of reading. II. THE MECHANICS OF READING