er to know how to deal with him. In some cases, hypnotism is a
valuable aid in treatment, and in many cases, ordinary normal suggestion
can be of considerable service. The state of mind of a sick person has
much to do with his recovery. The physician must know this and must know
how to induce the desired state of mind. Indeed, a patient's trouble is
often imaginary, exists in the mind only; in such cases, the treatment
should be wholly mental, _i.e._ through suggestion. Of course, the best
physicians know these facts and make use of them in their practice, but
preparation for this aspect of their work should be a regular part of
their medical education. They should not be left to learn these facts
from their practice as best they may, any more than they should be
expected to learn their physiology and anatomy in this way.

=Law.= The service of psychology to law can be very great, but owing to
the necessary conservatism of the courts, it will be a long time before
they will make much use of psychological knowledge. Perhaps the greatest
service will be in determining the credibility of evidence. Psychology
can now give the general principles in this matter. Witnesses go on the
stand and swear to all sorts of things as to what they heard and saw and
did, often months and even years previously. The expert clinical
psychologist can tell the court the probability of such evidence being
true. Experiments have shown that there is a large percentage of error
in such evidence. The additional value that comes from the oath has been
measured. The oath increases the liability of truth only a small
percentage.

Experiments have also shown that one's feeling of certainty is no
guarantee of truth. Sometimes the point we feel surest about is the one
farthest from the truth. In fact, feeling sure of a thing is no
guarantee of truth.

In a particular case in court, the psychologist can determine the
reliability of the evidence of a particular witness and enable the judge
and the jury to put the proper value on such witness's testimony. For
example, a witness may swear to a certain point involving the estimation
of time and distance. The psychologist can measure the witness's
accuracy in such estimates, often showing that what the witness claims
to be able to do is an impossibility. A case may hinge on whether an
interval of time was ten minutes or twelve minutes, or whether a
distance was three hundred or four hundred feet. A witness m