Note carefully the distinction between mental _ability_ and mental _capacity_. Mental ability in any individual is always less than his mental capacity. If, therefore, the mental ability as determined by scientific tests reaches the highest point on the scale of measurement, whatever that may be, it follows that the mental capacity of the individual making a perfect score is even greater than the scale is designed to measure, and how much greater can only be determined by setting up new tests based upon higher standards. The result of any scientific test simply indicates the wealth of nerve connections that are ready to be made when the stimulus necessary to their establishment is applied. It must be understood that no one having a sound claim to the possession of scientific knowledge can contend that there are tests in existence that actually measure with complete precision the inherited as distinguished from the acquired mental characteristics. It is not conceded, however, that such precise measurements cannot be made if at any time it becomes necessary or desirable to do so. For all practical present-day purposes it is sufficient that psychological tests shall measure mental qualities which are manifested by the individual’s ability to express them by action or speech. The classification of individuals relative to one another and with reference to the possession of a particular mental ability or group of abilities is, therefore, necessarily based upon their relative ability to express in some intelligible and unmistakable fashion their mental power and qualities. Back of this power of expression may lie hidden and undreamed-of capacities of which the individual himself may be vaguely conscious but of which he can give no outward manifestation. It may be, for example, that an individual is gifted with unusual powers of perception through the eyes, ears, and the senses of touch, smell, and taste but that he is deficient in nerve fibres and connections controlling the voluntary muscles by which human beings translate sensations into action and speech. This is hardly likely, as a physiological fact, to occur; the individual born with rich nerve endings in one part of the physical body is more likely to have a proportionate supply of nerve endings in all other parts of the body than to be deficient in one part and amply supplied in another. As rare exceptions, however, there are individuals who in infancy have, through accident or disease, lost certain groups of nerve connections while retaining unusually rich groups in other parts of the body. There is, of course, the most famous case in modern history, that of Helen Keller, whose auditory and optical nerve connections were lost through disease in early infancy, but whose unusual inherent mental capacity has been able to demonstrate itself through other and extraordinary means as a result of training and education.