Substance is something which can exist as subject and never as mere predicate(kB150)
The schema of substance is permanence of the real in time, that is, the representation of the real as a substrate of empirical determination of time in general, and so as abiding while all else changes(kB183)
The unity of experience would never be possible if we were willing to allow that new things, that is, new substances, could come into existence(kB230)
Coming to be does not concern substance, which does not come to be, but its state. It is, therefore, only alteration, not a coming to be out of nothing(kB252)
Thus the coexistence of substances in space cannot be known in experience save on the assumption of their reciprocal interaction. This is therefore the condition of the possibility of the things themselves as objects of experience(kB258)
The objects of the outer senses and the object of inner sense(the soul)(The formal condition of their intuition being in the case of the latter, time only, and in the case of the former, also space) differ from each other not inwardly but only in so far as one appears outwardly to the other, so that what, as thing in itself underlies the appearance of matter perhaps may not be so heterogeneous in character and the only question that remains is how in general a communion of substances is possible(k381)