But if they mean that there is void, not separately existent, but still present in the rare, this is less impossible; yet, first, the void turns out not to be a cause of all movement, but only of movement upwards (for the rare is light, which is the reason why they say fire is rare); second, the void turns out to be a cause of movement not as that in which it takes place, but in that the void carries things up as skins by being carried up themselves carry up what is continuous with them.