2Thales thought that water was the first principle of all things. Heraclitus, the Ephesian, who, on account of the obscurity of his writings, was called σκοτεινὸς by the Greeks, maintained a similar doctrine in respect of fire. Democritus, and his follower Epicurus, held similar opinions with regard to atoms; by which term is understood such bodies as are incapable of being cut asunder, or, as some say, of further division. To water and fire the philosophy of the Pythagoreans added air and earth. Hence Democritus, though loosely expressing himself, seems to have meant the same thing, when he calls the elements indivisible bodies; for when he considers them incapable of corruption or alteration, and of eternal duration and infinite solidity, his hypothesis makes the particles not yet so connected as to form a body.
2Since, therefore, all bodies consist of and spring from these elements, and in the great variety of bodies the quantity of each element entering into their composition is different, I think it right to investigate the nature of their variety, and explain how it affects the quality of each in the materials used for building, so that those about to build may avoid mistakes, and be, moreover, enabled to make a proper choice of such materials as they may want.