But let it be understood how we came to say that body passing through and through another body must produce disintegration, while we make qualities pervade their substances without producing disintegration: the bodilessness of qualities is the reason. Matter, too, is bodiless: it may, then, be supposed that as Matter pervades everything so the bodiless qualities associated with it — as long as they are few — have the power of penetration without disintegration. Anything solid would be stopped either in virtue of the fact that a solid has the precise quality which forbids it to penetrate or in that the mere coexistence of too many qualities in Matter [constitutes density and so] produces the same inhibition.
If, then, what we call a dense body is so by reason of the presence of many qualities, that plenitude of qualities will be the cause [of the inhibition].
If on the other hand density is itself a quality like what they call corporeity, then the cause will be that particular quality.
This would mean that the qualities of two substances do not bring about the mixing by merely being qualities but by being apt to mixture; nor does Matter refuse to enter into a mixing as Matter but as being associated with a quality repugnant to mixture; and this all the more since it has no magnitude of its own but only does not reject magnitude.