Now the human soul has the most important power of all other animals, that of grasping intelligibles or universals. It is also able to discriminate between good and evil in conduct, moral, political and economic. The human soul, therefore, has, it seems, two powers, _theoretical_ and _practical_. With the former it understands the simple substances, known as angels in the Bible and as "secondary causes" and "separate intelligences" among the philosophers. By this means the soul rises gradually to its perfection. With the practical reason it attends to noble and worthy conduct. All the other powers of the soul must be obedient to the behests of the practical reason. This in turn is subservient to the theoretical, putting its good qualities at the disposition of the speculative reason, and thus helping it to come into closer communion with the simple substances, the angels and God. This is the highest power there is in the _world of nature_. We must now show that the rational power in man is neither itself body nor is it a power residing in a corporeal subject. That it is not itself body is quite evident, for we have proved that the lower souls too, those of animals and plants, are not corporeal. But we must show concerning the rational power that it is independent of body in its activity. This we can prove in various ways. One is by considering the object and content of the reason. Man has general ideas or universal propositions. These are not divisible. An idea cannot be divided into two halves or into parts. Reason in action consists of ideas. Now if reason is a power residing in a corporeal subject, it would be divisible like the latter. Take heat as an example. Heat is a corporeal power, _i. e._, a power residing in a body. It extends through the dimensions of the body, and as the latter is divided so is the former. But this is evidently not true of general ideas, such as that a thing cannot both be and not be, that the whole is greater than its part, and so on. Hence the rational power is independent of body. Ibn Daud gives several other proofs, taken from Aristotle and Avicenna, to show that reason is independent, but we cannot reproduce them all here. We shall, however, name one more which is found in the "De Anima" of Aristotle and is based on experience. If the reason performed its thinking by means of a corporeal organ like the external senses, the power of knowing would be weakened when confronted with a difficult subject, and would thereby be incapacitated from exercising its powers as before. This is the case with the eye, which is dazzled by a bright light and cannot see at all, or the ear, which cannot hear at all when deafened by a loud noise. But the case of knowledge is clearly different. The more difficult the subject the more is the power of the reason developed in exercising itself therein. And in old age, when the corporeal organs are weakened, the power of reason is strongest.