Unquestionably the most usual and frequent ground for assuming a causal relation which is not immediately apparent is the similarity of the case in hand to other cases in which the causation has been ascertained. As it is the most frequent, so it is the most direct application of the fundamental Axiom of Causation, that Like causes in like conditions produce like effects, from which we obtain, by a logical process that is unknown to logicians, the immediate inference that Like effects in like conditions are due to like causes. It is by the application of this method not only that causation is most often established, but also that some of the most important discoveries of causes in the various sciences have been made. It is in perpetual use, both in the most recondite problems of science, and in the commonest affairs of daily life.