In the beginning of the book "On the Heavens," he proves[32\1] the world to be _perfect_, by reasoning of the following kind: "The bodies of which the world is composed are solids, and therefore have three dimensions: now three is the most perfect number; it is the first of numbers, for of _one_ we do not speak as a number; of _two_ we say _both_; but _three_ is the first number of which we say _all_; moreover, it has a beginning, a middle, and an end." [Note 32\1: De Cœlo, i. 1.] The reader will still perceive the verbal foundations of opinions thus supported. "The simple elements must have simple motions, and thus fire and air have their natural motions upwards, and water and earth have {73} their natural motions downwards; but besides these motions, there is motion in a circle, which is unnatural to these elements, but which is a more perfect motion than the other, because a circle is a perfect line, and a straight line is not; and there must be something to which this motion is natural. From this it is evident," he adds, with obvious animation, "that there is some essence of body different from those of the four elements, more divine than those, and superior to them. If things which move in a circle move contrary to nature, it is marvellous, or rather absurd, that this, the unnatural motion, should alone be continuous and eternal; for unnatural motions decay speedily. And so, from all this, we must collect, that besides the four elements which we have here and about us, there is another removed far off, and the more excellent in proportion as it is more distant from us." This fifth element was the "_quinta essentia_," of after writers, of which we have a trace in our modern literature, in the word _quintessence_. _Sect._ 3.--_Technical Forms of the Greek Schools._ WE have hitherto considered only the principle of the Greek Physics; which was, as we have seen, to deduce its doctrines by an analysis of the notions which common language involves. But though the Grecian philosopher began by studying words in their common meanings, he soon found himself led to fix upon some special shades or applications of these meanings as the permanent and standard notion, which they were to express; that is, he made his language _technical_. The invention and establishment of technical terms is an important step in any philosophy, true or false; we must, therefore, say a few words on this process, as exemplified in the ancient systems.