The Development of the Greek Conception of Nature

The theory of the elements (credited to Empedocles -- 490-35 B.C.)

According to the fundamental Greek theory of matter there are four basic elements:

Fire

Air

Water

Earth

None of these elements is to be identified with any object actually found in nature; those are all composites -- even a typical rock has, e.g., a bit of the element of water in it, and the water we drink has a bit of the element of earth, as is demonstrated by evaporating the water and noting the solids it leaves behind. Probably Empedocles, who was an atomist, thought of any given instance of matter as composed of myriad imperceptible particles; but it is not clear whether he held that all of the particles are identical or that some are different in size or shape from others.

The identification of the elements with the perfect solids (Plato -- 429-348 B.C.)

There are five "perfect" solids that can be constructed from identical regular polygons and are symmetrical at each vertex:

 # of faces shape of faces faces meeting at a vertex Tetrahedron (Fire) 4 triangle 3 Cube (Earth) 6 square 3 Octahedron (Air) 8 triangle 4 Dodecahedron (?) 12 pentagon 3 Icosahedron (Water) 20 triangle 5

Plato, in the Timaeus (53-6), identifies four of the perfect solids with the elements and creates a theory regarding the way in which the constituent triangles of the tetrahedron, octahedron and icosahedron may be divided and recombined to allow for the alteration of any of these elements into the others. He explains that "when many small bodies are dissolved into their triangles, by their total number, they can form one large mass of another kind." (54d) For example, "water, when divided by [i.e., cut apart by the sharp edges of] fire or by air, on re-forming, may become one part fire and two parts air; and a single volume of air divided becomes two of fire." (56d-e) This works out mathematically if we are counting the triangular faces: water (20) = two air (8+8) + fire (4), while air (8) = two fire (4+4). Plato considers the dodecahedron to have been an afterthought and does not integrate it into his account. In the Epinomis, a dialogue that may or may not be an authentic product of Plato's pen, the dodecahedron is associated with a fifth element called "ether." (984b)

Adaptation and elaboration of the theory of the elements (Aristotle (384-22 B.C.)

The four elements can be arranged in a chart in which each expresses the effects of a combination of four basic haptic (tangible) qualities (hot, cold, moist, dry) which are themselves paired in polar opposites (hot/cold, moist/dry). The contradictory properties do not combine, so we have the following possibilities:

 Moist Dry Cold Water Earth Hot Air Fire

According to Aristotle, transformations from one sort of matter into another take place in a cycle in which only one quality changes at a time. Thus earth may change into water (cold and dry changing to cold and moist) and the resulting water into air (cold and moist to hot and moist), but the transformation of earth into air cannot be effected in a single step. All of the cyclic transformations of this type are in principle reversible.

Each of the basic elements is itself just a form or manifestation of "prime matter," which is capable of taking on the various haptic qualities. Prime matter itself is not exactly a sort of matter; it can never be found in isolation, without properties, but has the potential to take on any consistent combination of physical properties.

Aristotle's theory of natural motion

In Aristotle we find the theory of the elements integrated with a theory of place and space. Place is not simply location, in the sense in which all locations are equal; different places have different intrinsic characters, and the various elements - and more complex bodies built up out of those elements - behave differently depending on the place in which they are found.

 Fire Absolutely light: moves upward, away from the center of the earth Air Relatively light: tends to move upward but more slowly than fire Water Relatively heavy: moves downward but less violently than earth Earth Absolutely heavy: moves downward toward the central point of the universe, not because that is where the earth is centered, but because earth and heavy, earthy bodies seek out that place by their intrinsic nature.

If all bodies were already in their natural places and no external force disturbed them, the world would be utterly still - no motion would take place, because everything would already be where it belongs.