The jury are now separated from their fellow-men during the whole case.  They are introduced into a world of new emotional values.  The ritual of the court, the voices and dress of judge and counsel, all suggest an environment in which the petty interests and impulses of ordinary life are unimportant when compared with the supreme worth of truth and justice.  They are warned to empty their minds of all preconceived inferences and affections.  The examination and cross-examination of the witnesses are carried on under rules of evidence which are the result of centuries of experience, and which give many a man as he sits on a jury his first lesson in the fallibility of the unobserved and uncontrolled inferences of the human brain.  The ‘said I’s,’ and ‘thought I’s,’ and ‘said he’s,’ which are the material of his ordinary reasoning, are here banished on the ground that they are ‘not evidence,’ and witnesses are compelled to give a simple account of their remembered sensations of sight and hearing.

The witnesses for the prosecution and the defence, if they are well-intentioned men, often find themselves giving, to their own surprise, perfectly consistent accounts of the events at issue.  The barristers’ tricks of advocacy are to some extent restrained by professional custom and by the authority of the judge, and they are careful to point out to the jury each other’s fallacies.  Newspapers do not reach the jury box, and in any case are prevented by the law as to contempt of court from commenting on a case which is under trial.  The judge sums up, carefully describing the conditions of valid inference on questions of disputed fact, and warning the jury against those forms of irrational and unconscious inference to which experience has shown them to be most liable.  They then retire, all carrying in their minds the same body of simplified and dissected evidence, and all having been urged with every circumstance of solemnity to form their conclusions by the same mental process.  It constantly happens therefore that twelve men, selected by lot, will come to a unanimous verdict as to a question on which in the outside world they would have been hopelessly divided, and that that verdict, which may depend upon questions of fact so difficult as to leave the practised intellect of the judge undecided, will very generally be right.  An English law court is indeed during a well-governed jury trial a laboratory in which psychological rules of valid reasoning are illustrated by experiment; and when, as threatens to occur in some American States and cities, it becomes impossible to enforce those rules, the jury system itself breaks down.[73]

[73] On the jury system see Mr. Wells’s Mankind in the Making, chapter vii.  He suggests the use of juries in many administrative cases where it is desirable that government should be supported by popular consent.