17. Since the substance changes its accidents, remaining the same itself, it follows that its existence is independent of the accidents. Abstracting, for the present, whether it can or cannot exist without any, I only affirm that none in particular is necessary to it. Here we must take note of the difference between substance in itself, and in the medium by which it is manifested to us, and placed in active or passive communication with us. The accidents are this medium; they are the transitory forms it puts on. How can we know the existence of bodies, except by sensations? The object of sensation is not substance in its inner nature, but only its qualities as affecting us. CHAPTER IV. RELATION OF CORPOREAL SUBSTANCE TO ITS ACCIDENTS. 18. In the idea of corporeal substance the idea of permanence is perfectly included, the idea of unity only imperfectly. The unity which we conceive in every corporeal substance is a factitious unity; since that which is constant is not one but an aggregate of many, as is proved by the divisibility of matter; out of every corporeal substance we may make many which will have the same right as the first to be called substances. A piece of wood is a substance; but we may slit it into several pieces which will be equally substances. These pieces, joined together, formed what are called _one_ substance; but it is clear that this unity was very imperfect, and was rather a union than a unity, and that if we consider it as _one_, it was in relation to the unity of effect which it produced in us, by the connection which it gave to our sensations and to the phenomena which resulted from it. 19. Hence, every corporeal substance involves multiplicity, or combination of the elements which compose it. Experience informs us that this combination is not permanent; there is, consequently, no corporeal substance which does not imply at least one modification, namely, the arrangement of its parts. Abstracting the changes which this modification may undergo, it can never be confounded with the substance: although the bodies might be presented constantly to our senses with the same arrangement of the parts, the permanent _being_ would be in the parts, not in their arrangement. The latter is something external which is added to the thing existing; there can be no union and combination without parts which are united and combined.