A 'generating force' was St Thomas's notion of the forces of chemistry. The fall of a stone he put down to removens prohibens, the support being removed, and the stone left free to gratify its natural appetite for rest on earth. The motion of the heavenly bodies he attributed, not without hesitation, to their being animated by a soul (Book II, Chap. LXX). Had he followed out the idea, here obscurely expressed, of the inertia of matter, he might have been led to divine the force of gravitation.
Of God and His Creatures: 1.97