It is obvious, therefore, that to question a man’s evidence is not always to call him a liar; in most cases it is rather to question the accuracy of his inferences from such part of the facts as he actually grasped.  In science no important observation is accepted until the experiments have been repeated and checked by other observers.  Indeed, most of the progress of science is due to the repetition of experiments by observers who notice some critical phenomena which their predecessors have missed.

With this qualification, that human observation is always fallible, good direct evidence is on the whole the most convincing evidence that you can use.  If you can establish a fact by the mouths of trustworthy witnesses, making your readers recognize that these witnesses had good opportunities of observation and a competent knowledge of the subject, you will generally establish your point.  In case of an accident in a street car it is the custom of many companies to require their conductors to take down immediately the names of a few of the most respectable-looking of the passengers, who may be called as witnesses in case of a lawsuit.  All the observations of science, and most of the facts brought before juries in courts of law, as well as the multitude of lesser and greater facts which we accept in everyday life, get their authority from this principle.

In the arguments of school and college you may not make much use of direct evidence, for they do not often turn on single, simple facts.  Even here, however, cases arise where you must call in the direct testimony of witnesses.  If you were arguing that secret societies should be abolished in a certain school, and wished to show that such societies had led to late hours, playing cards for money, and drinking, you would need direct evidence.  If you were arguing that the street railroad company of your city should be obliged to double track a certain part of its line, you would need direct evidence of the delays and crowding of cars with a single track.

When you are using direct evidence you should make it clear that the person from whom it comes is a competent witness, that he has been in a position to know the facts at first hand, and that, if necessary, he has had the proper training to understand their meaning.  In the case of an automobile accident a man who had never run a car would not be the best sort of witness as to the actions of the chauffeur, nor a man who had never sailed a boat as to what happened in a collision between two sailboats.  In a scientific matter the observations of a beginner would not carry weight as against those of a man who had used a microscope for many years.

The witness, too, must be shown to be free from bias, whether practical or theoretical.  It is a well-known fact that men differ greatly in the clearness of their eyesight in observing the stars, and that men who are gifted with exceptional eyesight may make valuable discoveries with inferior instruments; but if such a man has espoused a theory, say, as to the nature of the rings of Saturn, and is known to defend it passionately, his evidence concerning what he had seen is bound to be somewhat discounted.