Induction is defined as the legitimate inference of the more general, from the less general;--the general from the particular;--and (with more startling distinctness) of the Unknown from the Known. It is at once evident that, whatever may be the logical form into which this mode of inferring is thrown, there must in the nature of things be some ulterior principle to give it legitimacy. This principle, when raised to the rank and dignity of a philosophic postulate, is commonly known as the Law of Natural Uniformity. A law claiming such extensive dominion that one cannot help asking in what code, human or Divine, of reason or of experience, it was originally found written. Let us have recourse to the code of reason first. Euclid gives admirable instances of things true by necessity of reason. The moment we understand what right lines are, we see at once and for all time that two straight lines, infinitely prolonged, can never inclose a space. No one ever did see a mathematical line of any kind ("length without thickness"),--no one ever saw or conceived any real or ideal thing of infinite extent, neither can we think infinity at all. Yet the terms of the geometrical proposition carry their own evidence. We may sum the case, as Euler the mathematician put it. He finished a demonstration upon _Arches_ by saying, "All experience is contrary to this, but that is no reason for doubting its truth." Now, there appears nothing in the least resembling this case, in the conception of Natural Uniformity. No thinker can predicate substantial impossibility of the idea that Nature should ever be otherwise than Uniform. Suppose, then, we consider the code of Experience. Where shall we find the experience required? Ours is far short of universal, either in an absolute or an approximate sense. We are the children of to-day--yet the law wanted must be to all intents universal. It has been answered to this obvious requirement, that we enjoy the results of an experience constant and uniform, "coextensive not with the life of the single individual who employs them, but with the entire history of the human race."[154] But in what history is any such experience written? History in its _letter_, is full of events which contradict Nature's uniformity, of interruptions, marvels, miracles. For cattle to speak, is quite a common occurrence in Livy. An ordinary Roman would have been perplexed by the absence of signs and wonders; he would have felt it something to be accounted for. History tells us on every written page to believe in what seems impossible; and some writers on historical evidence, claim for it a greater amount of credibility whenever it testifies to the greater number of improbable incidents. For, do not writers of fiction deal in probabilities?[155]