Isidore’s conceptions in regard to number, indeed, deserve to be ranked closely after the theory of the four elements as affording to him “paths of intelligence” through the universe, material and immaterial. Both in the world at large and in the microcosm of man the harmony of “musical numbers” is an essential;[96] and number is also an essential factor in every part and aspect of the universe. “Take number from all things,” he says, “and all things perish.”[97] However, his idea of the importance of number in the world is equaled only by the vagueness with which he conceived its operations as a working principle. Here he takes absolute leave of the logic which, in his account of the four elements, he had already so often left behind. The best he could do, in describing the actual operation of this principle, was to make lists of instances in which the same number occurred, and no matter how unrelated the spheres of thought thus connected, to assume their close interrelation and explanation of one another. [96] _Etym._, 3, 23, 2. [97] _Etym._, 3, 4, 3. It is now clear that according to Isidore’s way of thinking, a fact belonging to one set of phenomena might be caused or explained by something totally different in another sphere. This being so, it was inevitable that there should be an effort to pass from the known to the unknown along the path thus suggested. When we reflect that, for the medieval thinker, there were three kinds of knowledge—namely, knowledge of the material, the moral, and the spiritual—and that they were in an ascending scale of value, it will appear equally inevitable that this effort to pass from the known to the unknown should be mainly an effort to pass from the material and obvious to the intangible and unseen, though more real, spiritual world. In this consideration we have the chief explanation of medieval allegory.[98] [98] The explanation suggested accounts for the prevalence of allegory in medieval times. Among the less comprehensive and not characteristically medieval causes for it must be reckoned the influence of the parables that are explained in the New Testament, the occasional grossness of Biblical characters and language which called for an interpretation that would remove offence and offer edification, the congenial activity which allegorizing offered to the pious mind, and, finally, the fact that by a clever use of allegorical interpretation some desired end might be obtained.