The fullness of the possibilities under which anything may be asserted is also of importance in this matter. The inference that a thing is impossible is generally made by most people in such wise that they first consider the details of the eventualities they already know, or immediately present. Then, when these are before them, they infer that the matter is quite impossible--and whether one or more different eventualities have missed of consideration, is not studied at all. Our kindly professor of physics once told us: “To-day I intended to show you the beautiful experiments in the interference of light--but it can not be observed in daylight and when I draw the curtains you raise rough-house. The demonstration is therefore impossible and I take the instruments away.” The good man did not consider the other eventuality, that we might be depended upon to behave decently even if the curtains were drawn. Hence the rule that a witness’s assertion that a thing is impossible must never be trusted. Take the simplest example. The witness assures us that it is impossible for a theft to have been committed by some stranger from outside. If you ask him why, he will probably tell you: “Because the door was bolted and the windows barred.” The eventuality that the thief might have entered by way of the chimney, or have sent a child between the bars of the window, or have made use of some peculiar instrument, etc., are not considered, and would not be if the question concerning the ground of the inference had not been put. We must especially remember that we criminalists “must not dally with mathematical truth but must seek historical truth. We start with a mass of details, unite them, and succeed by means of this union and test in attaining a result which permits us to judge the existence and the characteristics of past events.” The material of our work lies in the mass of details, and the manner and reliability of its presentation determines the certainty of our inferences. Seen more closely the winning of this material may be described as Hume describes it:[159] “If we would satisfy ourselves, therefore, concerning the nature of that evidence which assures us of matters of fact, we must inquire how we arrive at the knowledge of cause and effect. I shall venture to affirm as a general proposition which admits of no exception, that the knowledge of this relation is not, in any instance, attained by reasonings a priori; but arises entirely from experience, when we find that any particular objects are constantly conjoined with each other; ... nor can our reason, unassisted by experience, ever draw any inference concerning real existence and matter of fact.” In the course of his explanation Hume presents two propositions, (1) I have found that such an object has always been attended with such an effect. (2) I foresee that other objects which are in appearance similar, will be attended with similar effects.