The following account will make it plain that there is an absolutely light and an absolutely heavy body.And by absolutely light I mean one which of its own nature always moves upward, by absolutely heavy one which of its own nature always moves downward, if no obstacle is in the way.There are, I say, these two kinds of body, and it is not the case, as some maintain, that all bodies have weight.Different views are in fact agreed that there is a heavy body, which moves uniformly towards the centre.But is also similarly a light body.
For we see with our eyes, as we said before, that earthy things sink to the bottom of all things and move towards the centre.But the centre is a fixed point.If therefore there is some body which rises to the surface of all things-and we observe fire to move upward even in air itself, while the air remains at rest-clearly this body is moving towards the extremity.It cannot then have any weight.If it had, there would be another body in which it sank: and if that had weight, there would be yet another which moved to the extremity and thus rose to the surface of all moving things.In fact, however, we have no evidence of such a body.Fire, then, has no weight.Neither has earth any lightness, since it sinks to the bottom of all things, and that which sinks moves to the centre.That there is a centre towards which the motion of heavy things, and away from which that of light things is directed, is manifest in many ways.First, because no movement can continue to infinity.For what cannot be can no more come-to-be than be, and movement is a coming to-be in one place from another.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.But since that which sinks to the bottom of all things moves to the centre, necessarily that which rises to the surface moves to the extremity of the region in which the movement of these bodies takes place.For the centre is opposed as contrary to the extremity, as that which sinks is opposed to that which rises to the surface.This also gives a reasonable ground for the duality of heavy and light in the spatial duality centre and extremity.Now there is also the intermediate region to which each name is given in opposition to the other extreme.