On the Vital Principle/Book 3/Prelude to Chapter 13
There were four admitted elements, fire and air, earth and water, from which all things were supposed to be formed; and the object here is to shew that an animal cannot be homogeneous, connot be formed, that is, of only one element. Now, earth was supposed to give solidity, fire to be diffused through all living bodies, and thus there remained only air and water from which to constitute sentient organs; the earth was assigned, however, more particularly to the Touch, but, as Touch is perceptive of other qualities (as hot and cold) besides those of mere Touch, it could not be constituted of that element alone. The organs of relation, sight and hearing, were formed, principally, of water and air, which are the media for the transmission of visual and sonorous impressions; and the smell partook more of fire, as the Touch did of earth, yet not exclusively. The theory is superseded by the increase of our knowledge of external nature, but, in assuming elements, combination and proportion, it seems to typify, as it were, an atomic theory. Hippocrates[1], also, taught that the human body cannot be exclusively, either of air or fire, water or earth, or any single element; although, he adds, "I do not quarrel with such as think otherwise." Plato[2], likewise, “derived all things, so to say, from these four elements, in due proportion and relation to one another; so that what fire is to air, that air is to water, and water to earth, and each is, by affinity, united with others, to form whatever is visible and tangible.”