Since substance is of two kinds, the concrete thing and the formula (I mean that one kind of substance is the formula taken with the matter, while another kind is the formula in its generality), substances in the former sense are capable of destruction (for they are capable also of generation), but there is no destruction of the formula in the sense that it is ever in course of being destroyed; for there is no generation of it (the being of house is not generated, but only the being of this house), but without generation and destruction formulae are and are not; for it has been shown that no one produces nor makes these.