First in importance is an interpretation of the meaning of an Intelligence Quotient. Too often it is stated as a number and left as a number with the belief that somehow or other that is a tag which carries its own divine implication. Its importance lies in its diagnosis of power of adaptation, and it has a high correlation with the maximum possible rate of school progress. Just as a pure information test diagnoses the neural bonds that have been formed in any one field, so an intelligence test diagnoses the ability to form bonds, to meet a new situation and form satisfactory habits—power to learn. It may be thought of as a diagnosis of the neural chemistry of the individual. As such it is not concerned with the connections or quantity, but rather with the quality of the neural tissue. As an intelligence quotient is actual mental age divided by chronological age—which is the normal mental level of the child’s age-group—so it is the rate at which the child has progressed to mental maturity. It is his potential rate of progress. It is a division of what is by what normally would be. Then, when we use IQ we express the various degrees of power of adaptation due to various degrees of fitness of neural equipment to form bonds, by means of a diagnosis of the rate of formation of bonds which everyone forms sooner or later in an environment such as ours. It is conceivable that we might test this same power without testing the presence of such bonds at all. Such a test would detect directly the quality of the neural equipment irrespective of quantity or conformation. A ten-year-old child whose mental age is ten has progressed at the rate which is normal, and his IQ is 1.00. A very exceptional ten-year-old child whose mental age is fifteen has progressed just one and one half times as fast as the former, and his IQ is 1.50. Another exceptional ten-year-old child whose mental age is five has progressed at just one-half the rate of the first, and his IQ is .50. What we mean, then, by an Intelligence Quotient is the rate at which a child grows to the mental maturity of human beings in the world as it is. For purposes of presentation of a problem one can here assume (an hypothesis the value of which will here be determined) that each child can attain this rate of progress in each of the elementary school subjects. The degree to which this is true is the degree to which the IQ is a valid index of power to deal with school subjects. This assumes that inherited special disabilities in the school subjects are uncommon, that school progress is determined by the interplay of intelligence and environment, and that so-called interest characteristics which aid in development are the result of an earlier interplay of intelligence and environment. The degree to which educational product of children can be made to approach this intelligence will allow us to judge how far these factors are inherited, since differences that are removable must be learned, not innate.