Mental capacity tests may be devised that will measure certain mental qualities of an infant who has not yet learned to talk, and by thus providing a comparison between this particular child’s capacities and the average of children of the same age, enable parents and physicians to determine in what direction efforts looking toward its mental development may most helpfully be undertaken. Thus we may test the infant’s power of observation and perception of shapes, of colours, of sounds and familiar objects before it is able to talk, measuring these by standards derived from experience with similar tests applied to a large number of healthy, normal infants, and by this means determining whether the subject is above or below the normal average for its age and if so in what respects. At the other end of the scale of mental development, let us assume, is the possessor of the degree of Doctor of Philosophy from any of the great universities, since this is the principal degree the possession of which tends to show the possession of unusual mental powers, if not necessarily of wisdom. By applying to a large number of Ph.D.’s tests which are designed to require for their successful performance the utmost use of all their inherent mental abilities, and arriving at an average of performance by tabulating and comparing the degrees or percentages of perfection achieved by all of the individuals so tested, a standard is set up by which to measure the mental capacity of any individual or group of individuals of superior, or presumably superior, intelligence. By such a standard there may be measured also the mental capacity of men and women who have never seen the inside of a university, but whose education has been acquired in the course of their business and professional activities. This is so because _what is measured is not acquired knowledge, but the ability to acquire knowledge_, which is quite a different thing. The simplest way to measure the capacity of a circular tank is to pump it full of water and then measure the water as it is drawn off. But it would be absurd to contend that because there has never been any water pumped into the tank it is therefore impossible to determine how much water it would hold. And what the Doctor of Philosophy has got out of his university course is comparable to the water in the tank. The university may have assisted, and if its faculty were competent undoubtedly did assist him, in discovering earlier in life than he otherwise would have discovered the actual capacity of his mental tank. But there are probably as many men of equal mental capacity whose mental tanks have never been filled with the particular kind of intellectual fluid that the Ph.D. carries about with him, whose capacity there is no other means of measuring than by the application of mental tests based upon the known capacities of Doctors of Philosophy.