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.
Even official reports cannot be trusted without scrutiny.
The fact is that many things conspire to make an official report constrained and formal. There is the natural desire of every man to put the best face on things for himself as he sets his case before the government and the world; subordinates must be let off leniently; you must live with them, and it impairs comfort to have them sullen. To make a statement unpleasant to a superior might be construed as insubordination. The public welfare makes it imperative to tell a flattering tale. The temptation is constant to tell not quite the whole truth, and nothing but the truth. There are important suppressions of fact in the official records, none more so, perhaps, than as regards Chancellorsville.[17][!--Note--]
If you happen to be dealing with a historical matter, where the testimony comes from a more or less remote past, and the evidence is scrappy and defective, you must be still more careful. The great English historian, the late S.R. Gardiner, in his examination of the evidence on the Gunpowder Plot of 1605, wrote as follows about the difficulties of dealing with historical evidence:
It seems strange to find a writer so regardless of what is, in these days, considered the first canon of historical inquiry, that evidence worth having must be almost entirely the evidence of contemporaries who are in a position to know something about that which they assert. It is true that this canon must not be received pedantically. Tradition is worth something, at all events when it is not too far removed from its source. If a man whose character for truthfulness stands high, tells me that his father, also believed to be truthful, seriously informed him that he had seen a certain thing happen, I should be much more likely to believe that it was so than if a person, whom I knew to be untruthful, informed me that he had himself witnessed something at the present day. The historian is not bound, as the lawyer is, to reject hearsay evidence, because it is his business to ascertain the truth of individual assertions, whilst the lawyer has to think of the bearing of the evidence not merely on the case of the prisoner in the dock, but on an unrestricted number of possible prisoners, many of whom would be unjustly condemned if hearsay evidence were admitted. The historian is, however, bound to remember that evidence grows weaker with each link of the chain. The injunction, "Always leave a story better than you found it," is in accordance with the facts of human nature. Each reporter inevitably accentuates the side of the narrative which strikes his fancy, and drops some other part which interests him less. The rule laid down by the late Mr. Spedding, "When a thing is asserted as a fact, always ask who first reported it, and what means he had of knowing the truth," is an admirable corrective of loose traditional stories.
A further test has to be applied by each investigator for himself. When we have ascertained, as far as possible, on what evidence our knowledge of an alleged fact rests, we have to consider the inherent probability of the allegation. Is the statement about it in accordance with the general workings of human nature, or with the particular working of the nature of the persons to whom the action in question is ascribed? Father Gerard,[18][!--Note--] for instance, lavishly employs this test. Again and again he tells us that such and such a statement is incredible, because, amongst other reasons, the people about whom it was made could not possibly have acted in the way ascribed to them. If I say in any of these cases that it appears to me probable that they did so act, it is merely one individual opinion against another. There is no mathematical certainty on either side. All we can respectively do is to set forth the reasons which incline us to one opinion or another, and leave the matter to others to judge as they see fit.
It will be necessary hereafter to deal at length with father Gerard's attack upon the evidence, hitherto accepted as conclusive, of the facts of the plot. A short space may be allotted to the reasons for rejecting his preliminary argument, that it was the opinion of some contemporaries, and of some who lived in a later generation, that Salisbury contrived the plot in part, if not altogether. Does he realize how difficult it is to prove such a thing by any external evidence whatever? If hearsay evidence can be taken as an argument of probability, and in some cases of strong probability, it is where some one material fact is concerned. For instance, I am of opinion that it is very likely that the story of Cromwell's visit to the body of Charles I on the night after the king's execution is true, though the evidence is only that Spence heard it from Pope, and Pope heard it, mediately or immediately, from Southampton, who, it is alleged, saw the scene with his own eyes. It is very different when we are concerned with evidence as to an intention necessarily kept secret, and only exhibited by overt acts in such form as tampering with documents, suggesting false explanation of evidence, and so forth. A rumor that Salisbury got up the plot is absolutely worthless; a rumor that he forged a particular instrument would be worth examining, because it might have proceeded from some one who had seen him do it.[19][!--Note--]
While it is rare to find a man of whom it may justly be said that there is no partition between his memory and his imagination, yet there are few of us who can be sure of facts in past matters which touch our feelings. We cannot help to some degree reconstructing events as they fade away into the past: we forget those parts of an event which did not at the time sharply touch our imagination, and those which did move us take on an overshadowing importance. Therefore the further away the events which the evidence is to reconstruct, the more care we must take to scrutinize it to see if there are signs of bias.
To test the value of direct evidence, therefore, as to single and simple facts, consider whether the evidence comes from a specifically named source, whether there is any likelihood that the witness may have been honestly deceived in his observation, whether he had a good opportunity to know the facts and a sufficient knowledge of the subject about which he is giving evidence, and, finally, whether he was reasonably free from bias in the matter.