So, the lawyer, too, must suppose, although we lawyers have nothing to do with figures, that he knows nothing a priori, and must construct his inferences entirely from experience. And hence we must agree that our premises for such inferences are uncertain, and often subject to revision, and often likely, in their application to new facts, to lead to serious mistakes, particularly if the number of experiences from which the next moment is deduced, are too few; or if an unknown, but very important condition is omitted.
These facts must carefully be kept in mind with reference to the testimony of experts. Without showing ourselves suspicious, or desirous of confusing the professional in his own work, we must consider that the progress of knowledge consists in the collection of instances, and anything that might have been normal in 100 cases, need not in any sense be so when 1000 cases are in question. Yesterday the norm may have been subject to no exception; to-day exceptions are noted; and to-morrow the exception has become the rule.
Hence, rules which have no exceptions grow progressively rarer, and wherever a single exception is discovered the rule can no longer be held as normative. Thus, before New Holland was discovered, all swans were supposed to be white, all mammals incapable of laying eggs; now we know that there are black swans and that the duck-bill lays eggs. Who would have dared to assert before the discovery of the X-ray that light can penetrate wood, and who, especially, has dared to make generalizations with regard to the great inventions of our time which were not afterwards contradicted by the facts? It may be that the time is not too far away in which great, tenable and unexceptionable principles may be posited, but the present tendency is to beware of generalizations, even so far as to regard it a sign of scientific insight when the composition of generally valid propositions is made with great caution. In this regard the great physicians of our time are excellent examples. They hold: “whether the phenomenon A is caused by B we do not know, but nobody has ever yet seen a case of A in which the precedence of B could not be demonstrated.” Our experts should take the same attitude in most cases. It might be more uncomfortable for us, but certainly will be safer; for if they do not take that attitude we are in duty bound to presuppose in our conclusions that they have taken it. Only in this wise, by protecting ourselves against apparently exceptionless general rules, can our work be safely carried on.