the Cauſe that turns the Planets, and keeps them in their Orbits; and on thoſe Cauſes which make all Bodies here below deſcend towards the Surface of the Earth.
The Syſtem of Des Cartes explain'd and improv'd ſince his Time, ſeem'd to give a plauſible Reaſon for all thoſe Phænomena; and this Reaſon ſeem'd more juſt, as 'tis ſimple, and intelligible to all Capacities. But in Philoſophy, a Student ought to doubt of the Things he fancies he underſtands too eaſily, as much as of thoſe he does not underſtand.
Gravity, the falling of accelerated Bodies on the Earth, the Revolution of the Planets in their Orbits, their Rotations round their Axis, all this is mere Motion. Now Motion can't perhaps be conceiv'd any otherwiſe than by Impulſion; therefore all thoſe Bodies muſt be impelled. But by what are they impelled? All Space is full, it therefore is fill'd with a very ſubtile Matter, ſince this is imperceptible to
