The human body corresponds to the corporeal world, and is similar to it in its nature and matter. Man's body is subject to genesis and decay like other objects. It is composed of the elements and returns to them. It has in it the nature of minerals, plants and animals. It has the power of growth, sustenance and reproduction like plants. Man is like animal in having motion and sensation. He has the spirited power and the appetitive like other animals. His body is perfect because it has resemblances to all kinds of plants and animals. His body as a whole resembles great trees, his hair is like grass and shrubs. Animals have various qualities according to the relation of the animal soul to the body. Thus the lion has strength, the lamb meekness, the fox shrewdness, and so on. Mankind includes all of these qualities. In the same way various animals have various instincts resembling arts, such as the weaving of the spider, the building of the bird and the bee, and so on. They also subsist on various foods. Man alone combines all arts and all kinds of food. The human body has three dimensions like inanimate bodies. It is also similar to the bodies of plants and animals, and at the same time is distinguished alone among animals by its erect position. This is due to the fact that man's nature is proportionate, and his body is purer and finer than other bodies. Thus we see when oil is pure, its flame rises in a straight line; when the oil is impure the flame is not straight. Another thing proving that man's nature is superior to that of other animals is that the latter live in that element which is akin to their constitution--fish in water, birds in air, quadrupeds on land. Man alone can inhabit all three. Another reason for man's erect position is that he is a plant originating in heaven. Hence his head, which is the root, faces heaven.[171] Man has three souls, a plant soul, an animal soul and a rational soul. He must have a plant soul to account for the fact that man grows like other plants and dies like them. For if he can grow without a plant soul, plants can do the same. And if this too is granted, then there is no reason why mountains and stones should not grow also. Again, if man can grow without a plant soul, he can live without an animal soul, and know without a rational soul, which is absurd.