Aristotle on Elements
Scientific Thought I
The nature of heavenly bodies (On the heavens I 2)
Aristotle’s argument (in my words)
1 The motion of the heavenly bodies is continuous and eternal,
whereas unnatural motion is the quickest to stop. Therefore,
the motion of heavenly bodies is natural.
2 The motion of the heavenly bodies is circular, whereas the
natural motion of all bodies on Earth is straight (down or up).
Therefore, heavenly bodies are composed of something
different to anything on Earth.
If . . . the movement of the rotating bodies about the centre is
unnatural, it would be remarkable and indeed quite inconceivable
that this movement alone should be continuous and eternal, given
that it is unnatural. At any rate the evidence of all other cases goes
to show that it is the unnatural which quickest passes away . . .
Therefore, we may infer with confidence that there is something
beyond the bodies that are about us on this earth, different and
separate from them; and that the superior glory of its nature is
proportionate to its distance from this world of ours. [269b6]
Contrast with Anaxagoras
Anaxagoras believed that everything was originally mixed,
Mind started the mixture rotating, this caused the ingredients
to separate, and we see this rotation still in the heavens.
So Anaxagoras thought the circular motion of the heavenly
bodies was caused by Mind, not natural to them.
How a follower of Anaxagoras might criticize Aristotle’s
The heavenly bodies don’t move continuously and eternally;
initially they didn’t move.
Unnatural motion isn’t the quickest to stop. If you stir water
in a large pot, it keeps rotating for a long time (forever if you
keep stirring it), whereas falling stones don’t fall for long.
The existence of elements (On the heavens III 3)
An element . . . is a body into which other bodies may be analysed,
present in them potentially or in actuality . . . , and not itself
divisible into bodies different in form. [302a15]