Aristotle’s celestial spheres were supposed to generate species, conserve things in being, impart motion to things on earth, etc.. How were they supposed to do this? Surely there must be some sort of mechanism of action, right? The question is crucial since to invoke a mechanism means placing the heavens and the earth in an interaction, which utterly changes the Aristotelian account. Aristotle’s stars did not interact with corruptible things – there was one and only one direction of action. This is well-known and well-exemplified by Aristotle’s understanding of gravity (heaviness or weight) in which the earth acted on stones, but the stones did not pull back on the earth. If one assumed the moon was a large falling stone, the center of the earth would pull on it, but it would not pull back on the earth.
But nature has no simple actions, but only interactions. This is, it seems, a necessary postulate and consequence of mechanist explanations.