Subject line becomes time, rather than spatial extension. Pictures on p. 299 show various kinds of motion. The 'Merton Rule' or 'mean speed theorem'. See picture on p. 300. This was done as pure mathematics.
All motion requires a mover.
In natural motion, the mover does not seem sufficiently distinct from the thing moved. Why does an object keep moving when you throw it. This is distinct from the motion itself. This could explain motion of the heavens. Criticized by Philoponus, on empirical grounds.