Inasmuch as the question deals only with the scarcity of correct knowledge of proximate causes, we shall set aside the fact that lawyers themselves make such mistakes, which may be avoided only by careful self-training and cautious attention to one’s own thoughts. But we have at the same time to recognize how important the matter is when we receive long series of inferences from witnesses who give expression only to the first and the last deduction. If we do not then examine and investigate the intermediary links and their justification, we deserve to hear extravagant things, and what is worse, to make them, as we do, the foundation of further inference. And once this is done no man can discover where the mistake lies. If again an inference is omitted as self-evident (cf. the case of gravity, in falling down stairs) the source of error and the difficulty lies in the fact that, on the one hand, not everything is as self-evident as it seems; on the other, that two people rarely understand the same thing by “self-evident,” so that what is self-evident to one is far from so to the other. This difference becomes especially clear when a lawyer examines professional people who can imagine offhand what is in no sense self-evident to persons in other walks of life. I might cite out of my own experience, that the physicist Boltzmann, one of the foremost of living mathematicians, was told once upon a time that his demonstrations were not sufficiently detailed to be intelligible to his class of non-professionals, so that his hearers could not follow him. As a result, he carefully counted the simplest additions or interpolations on the blackboard, but at the same time integrated them, etc., in his head, a thing which very few people on earth can do. It was simply an off-hand matter for this genius to do that which ungenial mortals can not. This appears in a small way in every second criminal case. We have only to substitute the professionals who appear as witnesses. Suppose, e.g., that a hunter is giving testimony. He will omit to state a group of correlations; with regard to things which are involved in his trade, he will reach his conclusion with a single jump. Then we reach the fatal circle that the witness supposes that we can follow him and his deductions, and are able to call his attention to any significant error, while we, on the other hand, depend on his professional knowledge, and agree to his leaping inferences and allow his conclusions to pass as valid without knowing or being able to test them.