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 may swear positively to one or both of these points. The psychologist can show the court the limitations of the witness in making such estimates. Psychology can be of service in the examination of the criminal himself. Through association tests and in other ways, the guilt or innocence of the prisoner can often be determined, and his intellectual status can also be determined. The prisoner may be insane, or feeble-minded, or have some other peculiar mental disorder. Such matters fall within the realm of psychology. After a prisoner has been found guilty, the court should have the advice of the clinical psychologist in deciding what should be done with him. It should be added that the court and not the attorneys should make use of the psychologist. Whenever a psychologist can be of service in a case in court, the judge should summon such assistance, just as he should if expert chemical, physical, physiological, or anatomical knowledge should be desired. A knowledge of human nature can be of much service to society in the prevention of crime. This will come about from a better knowledge of the psychological principles of habit-formation and moral training, through a better knowledge of how to control human nature. A large percentage of all crime, perhaps as much as forty per cent, is committed by feeble-minded people. Now, if we can detect these people early, and give them the simple manual education which they are capable of receiving, we can keep them out of a life of crime. Studies of criminals in reform schools show that the history of many cases is as follows: The person, being of low mentality, could not get on well at school and therefore came to dislike school, and consequently became a truant. Truancy led to crime. Crime sent the person to the court, and the court sent the person to the state reformatory. The great duty of the state is the prevention of crime. Usually little can be done in the way of saving a mature criminal. We must save the children before they become criminals, save them by proper treatment. Society owes it to every child to do the right thing for him, the right thing, whether the child is an idiot or a genius. Merely from the standpoint of economy, it would be an immense saving to the state if it would prevent crime by the proper treatment of every child.