sm and skepticism; namely, the fact
that every one, whether psychologically trained or not, acquires in the
ordinary experiences of life a certain degree of expertness in the
observation and interpretation of mental traits. The possession of this
little fund of practical working knowledge makes most people slow to
admit any one's claim to greater expertness. When the astronomer tells
us the distance to Jupiter, we accept his statement, because we
recognize that our ordinary experience affords no basis for judgment
about such matters. But every one acquires more or less facility in
distinguishing the coarser differences among people in intelligence,
and this half-knowledge naturally generates a certain amount of
resistance to the more refined method of tests.

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 haemoglobin. 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