[The manner of their generation was as follows:— The four elements are solid bodies, and all solids are made up of plane surfaces, and all plane surfaces of triangles. All triangles are ultimately of two kinds,—i. e. the rectangular isosceles, and the rectangular scalene. The rectangular isosceles, which has but one form, and that one of the many forms of scalene which is half of an equilateral triangle were chosen for making the elements.] In the first place, then, as is evident to all, fire and earth and water and air are bodies.