The general organization of the matter treated by Isidore in the _De Natura Rerum_ is worth noticing. The preface quoted above indicates that the order of treatment is to follow the order of creation. The first topic, therefore, suggested by the creation of light, we should expect to be the phenomenon of light. Instead of this it is the day, in the calendar sense, that is described, with the natural sequel of the week, month, and year as collections of days. This section really constitutes a brief account of the elements of chronology. Next created are the heavens; so we have next astronomy, presented in a condensed form, to which are appended a few chapters on meteorological matters, such as thunder, clouds, the rainbow, wind, and finally pestilence, which comes in appropriately here as being “a corruption of the air”. The topic next in order, following the first chapter of Genesis, is the sea; and after that, the dry land. It should be noted that this view of the physical universe according to the order of its creation, corresponds roughly to the analysis of matter into the four elements, fire, air, water, earth. As will be shown later, such correspondences are an important factor in the intellectual outlook of the time. This was the kind of mental connection with which people were familiar.[32] [32] See p. 64. The _Liber Numerorum_ contains nothing arithmetical in the modern sense of the word, in spite of Braulio’s statement that in it Isidore “touched on the science of arithmetic”.[33] Its fuller title is “The book of the numbers which occur in the Holy Scriptures”, and the body of the book is taken up with the mystic significance of each number from one to twenty, omitting seventeen, and also of twenty-four, thirty, forty, forty-six, fifty, and sixty. The method of treatment indicates an advanced mysticism of numbers. The book is not so much an attempt to show the significance of numbers occurring in particular connections, as it is a generalized guide to their mystical interpretation, laying down rules to govern the interpretation of each number, no matter where it occurs. It should be remarked that this was really “the science of number” of the dark ages, and that Braulio’s use of the term “arithmetic” as applying to it was in accordance with the best usage of the time.[34] [33] See p. 24. [34] See p. 126. The _Allegoriae_ is of a character similar to the _Liber Numerorum_. It contains in brief form the principal allegories which were read into the books of the Old and the New Testaments, and is evidently meant to constitute a sort of reference book for Scriptural allegory. It possesses little interest.