Among the writers of antiquity who dealt with physical geography only three can be said to have influenced twelfth- and early thirteenth-century thought to any marked degree. These were, first and foremost, Aristotle, the substance of whose _De caelo_ and _Meteorology_ had reached the West before the year 1187 through the borrowings and plagiarisms of later scholars and after that time could be read in translations from the Greek and Arabic. In the second place, Seneca’s _Quaestiones naturales_ was popular before the direct influence of the _De caelo_ and _Meteorology_ began to be felt. In the third place, as we have seen, the Elder Pliny’s _Historia naturalis_ was not only widely read in the original, but also much that it contained was familiar through the intermediary channels of Solinus, Isidore, Martianus Capella, and others. Aristotle, however, was the fundamental authority, for a large portion of the material in the books of the two Latin authors came from his treatises. THE FOUR ELEMENTS Most ancient authorities believed that the universe is composed of four elements, fire, air, water, and earth, arranged in concentric spheres. Theoretically, according to this view, the sphere of water should entirely enclose the earth. Practical observation shows that it covers the lower levels of the earth’s surface only. How to reconcile the theoretical conception with observed facts was a problem which, as we shall see, greatly puzzled geographers and physicists during the later Middle Ages.[72] According to Aristotle the four elements, under the control of the heavenly bodies and through their interaction upon each other, produce all the physical phenomena of the atmosphere, sea, and earth.[73] Working from this axiom, he, and all the ancient writers who dealt with the subject, attempted to explain winds, tides, earthquakes, and other occurrences of nature; but there was little agreement among them as to the manner in which these interactions were manifested. Though there were many theories, the actual matters under discussion were not very numerous. Only the most striking and unusual happenings—such as tides, earthquakes, and floods—attracted attention, and we find almost no trace of a minute and careful observation or even of a superficial understanding of those imperceptibly slow natural forces which modern geology recognizes as having fashioned mountains, rivers, and seas.