Perhaps the most important but obscure argument in the whole book is Aristotle's demonstration of the immortality of the thinking part of the human soul, also in Chapter V. Taking a premise from his "Physics", that as a thing acts, so it is, he argues that since the active principle in our mind acts with no bodily organ, it can exist without the body. And if it exists apart from matter, it therefore cannot be corrupted. And therefore there exist a mind which is immortal. As to what mind Aristotle is referring to in chapter V (i.e. divine, human or a kind of world soul), has represented for centuries a hot topic of discussion. The most likely is probably the interpretation of Alexander of Aphrodisias, likening Aristotle immortal mind to an impersonal activity, ultimately represented by God.