“Secondly, like the upward movement of fire, the downward movement of earth and all heavy things makes equal angles on every side with the earth's surface: it must therefore be directed towards the centre. Whether it is really the centre of the earth and not rather that of the whole to which it moves, may be left to another inquiry, since these are coincident.” (Gravitation)
On Generation and Corruption – do things come from causes, prime material, or alteration?
On the Soul – the kinds of souls possessed by living things. This does not mean soul in the religious sense but the character of the mind (ego?).