But in all this there is more probability than truth. For consider, friend Demetrius; when he saith that some bodies tend towards the midst, which is to say, downwards, the others from the midst, that is, upward, and a third sort move round about the midst, what does he mean by the midst? This cannot be understood in respect of a vacuum, there being no such thing in Nature, as he says himself; and, moreover, those that do allow it say that it can have no middle, no more than beginning and end; for beginning and end are extremities, but that which is infinite, everybody knows, is without an end. But supposing we should be necessitated to admit a middle in a vacuum, it is impossible to comprehend and imagine the difference in the motions of bodies towards it, because there is neither in this vacuum any power attractive of the body, nor in the bodies any inclination or affection to tend on all sides to this middle. And it is no less difficult to imagine that bodies can move of themselves towards an incorporeal place, or receive any motion from it. This middle then must be understood not locally, but corporeally. For this world being a mass and union consisting of different bodies joined together, this diversity of them must beget different motions from one another; which appears in that each of these bodies changing its substance does at the same time change its place. For subtilization and rarefaction dissipate the matter which springeth from the midst and ariseth upwards; whereas, on the contrary, condensation and constipation depress and drive it down towards the middle.