As the concepts of physics are essential for proving the existence of God, so are the principles of psychology of importance in showing that there are intermediate beings between God and the corporeal substances of the world. These are called in the Bible angels. The philosophers call them secondary causes.
Accordingly Ibn Daud follows his physical doctrines with a discussion of the soul. There is nothing new in his proof that such a thing as soul exists. It is identical with the deduction of Joseph Ibn Zaddik (_supra_, p. 134). Stone and tree and horse and man are all bodies and yet the last three have powers and functions which the stone has not, _viz._, nutrition, growth and reproduction. Horse and man have in addition to the three powers above mentioned, which they have in common with tree, the powers of sensation and motion and imagination, which plants have not. Finally man is distinguished above all the rest of animal creation in possessing the faculty of intelligence, and the knowledge of art and of ethical discrimination. All these functions cannot be body or the result of body, for in that case all corporeal objects should have all of them, as they are all bodies. We must therefore attribute them to an extra-corporeal principle; and this we call soul. As an incorporeal thing the soul cannot be strictly defined, not being composed of genus and species; but we can describe it in a roundabout way in its relation to the body. He then gives the Aristotelian definition of the soul as "the [first] entelechy of a natural body having life potentially" (_cf._ above, p. xxxv).
Like many of his predecessors who treated of the soul, Ibn Daud also finds it necessary to guard against the materialistic theory of the soul which would make it the product of the elemental mixture in the body, if not itself body. This would reduce the soul to a phenomenon of the body, or in Aristotelian terminology, an accident of the body, and would deprive it of all substantiality and independence, not to speak of immortality. How can that which is purely a resultant of a combination of elements remain when its basis is gone? Accordingly Ibn Daud takes pains to refute the most important of these phenomenalistic theories, that of Hippocrates and Galen. Their theory in brief is that the functions which we attribute to the soul are in reality the results of the various combinations of the four elementary qualities, hot, cold, moist, dry. The more harmonious and equable the proportion of their union, the higher is the function resulting therefrom. The difference between man and beast, and between animal and plant is then the difference in the proportionality of the elemental mixture. They prove this theory of theirs by the observation that as long as the mixture is perfect the activities above mentioned proceed properly; whereas as soon as there is a disturbance in the mixture, the animal becomes sick and cannot perform his activities, or dies altogether if the disturbance is very great. The idea is very plausible and a great many believe it, but it is mistaken as we shall prove.