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] 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]

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.