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Democritus's atoms: *
The primary movement of atoms as described by Democritus, comes from collisions with other atoms; their mutual resistance causes them to move away from one another when struck. He gave no “first cause” for this movement, though the prior collision with another atom accounts for the direction of each individual atomic motion. Some atoms will adhere or attach to each other. Such clusters of atoms moving in the infinite void form the world as a result of a circular motion that gathers atoms up into a whirl, creating clusters within it. However these worlds are impermanent; these creations & all the species inhabiting them arise from the collision of atoms moving about randomly in such a whirl. And likewise these worlds will disintegrate in time.
Democritus ascribed the causes of things to necessity & chance, or better ‘absence of purpose’. He needed to explain how the disorderly motion of atoms could produce an orderly cosmos where atoms are not just randomly scattered, but cluster to form masses of distinct types. He employs a mechanistic explanation: the ‘like to like’ tendency of nature. Just as animals of a kind cluster together, so atoms of similar kinds cluster by size & shape. He compares this to the sorting of pebbles carried by the tide. Thus ordered arrangement arise automatically, a by-product of the random collisions of bodies in motion. No attractive forces are needed to explain the sorting by the tide; orderly effects can be produced without goal-directed forces or purpose.
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Aristotle: *
for him science embraced mathematics & metaphysics as well as the empirical sciences; in his Physics, he establishes general principles of change that govern all natural bodies as well as motion, quantitative change, qualitative change and generation/death. Physics was a broad field including philosophy, sensory experience, memory, anatomy & biology. Key concepts include the structuring of the cosmos into concentric spheres, Earth at the centre & celestial spheres around it. The Earth consisted of earth, air, fire & water, all subject to change & decay while the celestial spheres were made of unchangeable aether (the 5th element). Terrestrial objects have natural motions determined by their dominant element: those of earth & water fall; those of air & fire, rise. Speed depends on weight & density of the medium. A vacuum cannot exist.