Full Idea
For Aristotle, aether and the four sublunary elements obey different physical laws. Aether moves naturally in a circle and, unlike its lower counterparts, is not a source of perishability. The four sublunary elements move naturally in straight lines.
Gist of Idea
Aether moves in circles and is imperishable; the four elements perish, and move in straight lines
Source
report of Aristotle (On the Heavens [c.336 BCE]) by Mary Louise Gill - Aristotle on Substance Ch.2
Book Reference
Gill,Mary Louise: 'Aristotle on Substance: Paradox of Unity' [Princeton 1989], p.67
A Reaction
I think it is anachronistic for Gill to talk of 'obeying' and 'laws'. She should have said that they have different 'natures'. We can be amused by Greek errors, until we stare hard at the problems they were trying to solve.