But in ordinary life, if a man or a woman has some mental quality which does not express itself in an action which other persons can see or hear and know about, then it is not socially important. It is of consequence only to the individual and it is of little social service to undertake to measure these obscure and unexpressed and inexpressible capacities, as they can never, until they find means of expression, affect the individual’s ability or efficiency in any occupation. It is not that these things cannot be measured. The case of Helen Keller is one demonstration that they can be measured. Anything whatever that makes a difference in the way different individuals act is conceivably measurable, although it may not at the present time be capable of exact calculation because it has not been worth anybody’s time and effort to undertake to measure it. To repeat, and possibly to make the preceding paragraphs more clear, let us recapitulate the different mental qualities to which reference has been made. First, mental _capacity_. This is what the individual has inherited. It is the size of the tank into which sensations, perceptions, all that makes up the sum of knowledge, are poured throughout his life, by his education and his experience. While this capacity in the case of any individual can doubtless be measured, it is not necessary to measure it precisely but merely to determine whether it is large enough for the purposes in view. Second, mental _ability_. This is the sum of experience and education within the limits of the individual’s mental capacity. It is represented by the individual’s ability to express himself in speech or action in the performance of any one of a number of specific acts. This mental ability can be quite definitely measured, and the possession of a certain degree of mental ability demonstrates the possession of a mental capacity greater than the ability which the individual has already reached. Third, _acquired knowledge_. It is not the purpose of tests of mental capacity to measure acquired knowledge, although for many purposes it is desirable to measure the individual’s acquired knowledge _in addition_ to his inherent ability, and in a still larger number of instances the most practical way of arriving at a fairly accurate estimate of an individual’s ability involves, among other tests, an examination into the extent of the knowledge which he has acquired through observation or training along lines definitely related to his particular occupation or pursuit in life.