There are some, however, who fancy that it is evident there is a vacuum, from the rare and the dense; for if there is not the rare and the dense, it is not possible for things to come together and be compressed. And if this does not take place, either, in short, there will not be motion, or the universe will overflow, as Xuthus said, or air and water will always be changed into the equal. I say, for instance, if air should be generated from a cup of water, at the same time as much water must be generated from equal air; or there must necessarily be a vacuum; for it is not otherwise possible that compression and co-extension can exist. If, therefore, they call the rare that which has many void separate spaces; it is evident, that if there cannot be a separate vacuum, as neither is there a place which has an interval of itself, neither can the rare subsist in this manner.
But if they say that there is not a separate vacuum, but that at the same time a vacuum is inherent, this indeed is less impossible. It will happen, however, in the first place, that a vacuum is not the cause of all motion, but only of that which is upward: for the rare is light; and on this account they say that fire is rare. In the next place, a vacuum will not be the cause of motion as that in which; but just as bladders, because they are carried upward, carry also that which adheres to them, in like manner a vacuum will have a power of carrying upward. But how is it possible that there can be lation of a vacuum, or a place of a vacuum? for there would be a vacuum of a vacuum, into which it would be carried. Again, how can they assign the cause why a heavy body tends downward? And it is manifest, if by how much the more rare a body is, and contains void spaces, by so much the more it will be impelled upward, so that if, in short, there is a vacuum, it will be most rapidly moved. Perhaps, however, it is impossible for a vacuum to be moved. And the reason is the same; because, as in a vacuum, all things are immoveable, so likewise a vacuum is immoveable: for the celerities of a vacuum and a solid cannot be compared with each other.
Since, however, we say that there is not a vacuum, and it is truly doubted of the rest, hence, either there will not be motion, unless there is condensation and rarefaction, or the universe will overflow, or there will always be an equal quantity of water from air, and of air from water: for it is evident that more air will be produced from water. It is necessary, therefore, unless there is compression, or that which adheres is expelled, that what is last must be made to overflow, or an equal quantity of air must in some other place be changed into water, in order that the whole bulk of the universe may be equal; or if this be not admitted, it will follow that nothing can be moved; for some body being always transferred, this will happen, unless it revolves in a circle. But lation is not always in a circle; but likewise in a right angle. These, therefore, through arguments of this kind, assert that there is a vacuum. We, however, say, from tlie things wliich are admitted, that there is one matter of contraries, viz. of the hot and the cold, and of other contrarieties; also that from what is in capacity, that which is in energy is produced; that matter is not separate, but is different in essence; and that the matter, for instance, of colour, of the hot and of the cold, is one in number. The matter also of body, of the great and of the small, is the same. But this is manifest: for when air is generated from water, the same matter, not assuming any thing else, is produced; but that which was in capacity becomes in energy. And again, in like manner, water is generated from air; a change being made at one time into magnitude from parvitude, and at another from parvitude into magnitude. In a similar manner, therefore, if air passes from a greater into a less quantity, and from a less into a greater, the matter which is in capacity becomes both: for as the same matter from being cold becomes hot, and from being hot, cold, because it was in capacity so likewise from being hot it becomes more hot, no part of the matter being made hot which was not hot before, when there was less heat. Just as if the circumference and convexity of a greater circle should become the circumference of a less, whether it be the same or another, the convexity would not be in any part which was not convex but straight; for it is not less or more by intermission. Nor can any magnitude of flame be assumed in which both whiteness and heat are not inherent. Thus, therefore, is the prior heat with reference to the posterior. So that the magnitude and parvitude of the sensible bulk is extended, the matter not assuming any thing; but because the matter is in capacity to both. Hence the same thing is both dense and rare, and there is one matter of these. The dense also is heavy; but the rare, light; for there are two things in each, that is, in the rare and the dense: for the heavy and the hard appear to be dense; and the contraries appear to be rare, viz. the light, and the soft. But the heavy and the hard are discordant in lead and iron. From what has been said, therefore, it is evident that there is not a separate vacuum, (whether simply considered, or as in that which is rare) nor a vacuum in capacity, unless some one in short, should be willing to call a vacuum that which is the cause of lation. But thus the matter of the heavy and light, so far as it is such, will be that vacuum; for the dense and the rare, according to this contrariety, are effective of lation; but according to the hard and the soft, they have the power of producing passion and impassivity, and not lation, but rather change according to quality. After this manner, therefore, let it be determined concerning a vacuum, how it subsists, and how it does not subsist.
