It should be evident, however, that we need more than the ability merely to distinguish a genius from a simpleton, just as a physician needs something more than the ability to distinguish an athlete from a man dying of consumption. It is necessary to have a definite and accurate diagnosis, one which will differentiate more finely the many degrees and qualities of intelligence. Just as in the case of physical illness, we need to know not merely that the patient is sick, but also why he is sick, what organs are involved, what course the illness will run, and what physical work the patient can safely undertake, so in the case of a retarded child, we need to know the exact degree of intellectual deficiency, what mental functions are chiefly concerned in the defect, whether the deficiency is due to innate endowment, to physical illness, or to faults of education, and what lines of mental activity the child will be able to pursue with reasonable hope of success. In the diagnosis of a case of malnutrition, the up-to-date physician does not depend upon general symptoms, but instead makes a blood test to determine the exact number of red corpuscles per cubic millimeter of blood and the exact percentage of hæmoglobin. He has learned that external appearances are often misleading. Similarly, every psychologist who is experienced in the mental examination of school children knows that his own or the teacher's estimate of a child's intelligence is subject to grave and frequent error. THE NECESSITY OF STANDARDS. In the first place, in order to judge an individual's intelligence it is necessary to have in mind some standard as to what constitutes normal intelligence. This the ordinary parent or teacher does not have. In the case of school children, for example, each pupil is judged with reference to the average intelligence of the class. But the teacher has no means of knowing whether the average for her class is above, equal to, or below that for children in general. Her standard may be too high, too low, vague, mechanical, or fragmentary. The same, of course, holds in the case of parents or any one else attempting to estimate intelligence on the basis of common observation.