The notion of “specialist” or “professional” must be applied in such instances not only to especial proficients in some particular trade, but also to such people as have by accident merely, any form of specialized knowledge, e.g., knowledge of the place in which some case had occurred. People with such knowledge present many a thing as self-evident that can not be so to people who do not possess the knowledge. Hence, peasants who are asked about some road in their own well known country reply that it is “straight ahead and impossible to miss” even when the road may turn ten times, right and left. Human estimates are reliable only when tested and reviewed at each instant; complicated deductions are so only when deduction after deduction has been tested, each in itself. Lawyers must, therefore, inevitably follow the rule of requiring explication of each step in an inference--such a requirement will at least narrow the limits of error. The task would be much easier if we were fortunate enough to be able to help ourselves with experiments. As Bernard[133] says, “There is an absolute determinism in the existential conditions of natural phenomena, as much in living as in non-living bodies. If the condition of any phenomenon is recognized and fulfilled the phenomenon must occur whenever the experimenter desires it.” But such determination can be made by lawyers in rare cases only, and to-day the criminalist who can test experimentally the generally asserted circumstance attested by witnesses, accused, or experts, is a rarissima avis. In most cases we have to depend on our experience, which frequently leaves us in difficulties if we fail thoroughly to test it. Even the general law of causation, that every effect has its cause, is formulated, as Hume points out, only as a matter of habit. Hume’s important discovery that we do not observe causality in the external world, demonstrates only the difficulty of the interpretation of causality. The weakness of his doctrine lies in his assertion that the knowledge of causality may be obtained through habit because we perceive the connection of similars, and the understanding, through habit, deduces the appearance of the one from that of the other. These assertions of the great thinker are certainly correct, but he did not know how to ground them. Hume teaches the following doctrine: