Full Idea
There are two types of necessity, one according to nature and impulse, the other by force and contrary to impulse. A stone travels upwards and downwards from different necessities.
Gist of Idea
A stone travels upwards by a forced necessity, and downwards by natural necessity
Source
Aristotle (Posterior Analytics [c.327 BCE], 94b38)
Book Reference
Aristotle: 'Posterior Analytics (2nd ed)', ed/tr. Barnes,Jonathan [OUP 1993], p.61