Metaphysics VII 15 1039b20-1040a8

• 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 not 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.
  
• For this reason also there is neither definition nor demonstration of sensible individual substances, because they have matter whose nature is such that they are capable both of being and of not being; for which reason all the individual substances of them are destructible.
  
• If then demonstration is of necessary truths and definition involves knowledge, and if, just as knowledge cannot be sometimes knowledge and sometimes ignorance, but the state which varies thus is opinion, so too demonstration and definition cannot vary thus, but it is opinion that deals with that which can be otherwise than as it it is, clearly there can neither be definition nor demonstration of sensible individuals. For perishing things are obscure to those who have knowledge of them, when they have passed from our perception; and though the formulae remain in the soul unchanged, there will no longer be either definition, or demonstration. Therefore when one of those who aim at definition defines any individual, he must recognize that his definition may always be overthrown; for it is not possible to define such things.