His refutation of the "accident" or "mixture" theory of the soul, as well as the subsequent discussion of the various functions, sensuous and rational, of the tripartite soul, are based upon Ibn Sina's treatment of the same topic, and we have already reproduced some of it in our exposition of Judah Halevi. We shall therefore be brief here and refer only to such aspects as are new in Ibn Daud, or such as we found it advisable to omit in our previous expositions. His main argument against the materialistic or mechanistic theory of the soul is that while a number of phenomena of the growing animal body can be explained by reference to the form of the mixture in the elementary qualities, not all aspects can be thus explained. Its growth and general formation may be the result of material and mechanical causes, but not so the design and purpose evident in the similarity, to the smallest detail, of the individuals of a species, even when the mixture is not identical. There is no doubt that there is wisdom here working with a purpose. This is soul. There is another argument based upon the visible results of other mixtures which exhibit properties that cannot be remotely compared with the functions we attribute to the soul. The animal and the plant exhibit activities far beyond anything present in the simple elements of the mixture. There must therefore be in animals and plants something additional to the elements of the mixture. This extra thing resides in the composite of which it forms a part, for without it the animal or plant is no longer what it is. Hence as the latter is substance, that which forms a part of it is also substance; for accident, as Aristotle says, is that which resides _in_ a thing but not as forming a part of it. We have now shown that the soul is substance and not accident. We must still make clear in which of the four senses of the Aristotelian substance the soul is to be regarded. By the theory of exclusion Ibn Daud decides that the soul is substance in the sense in which we apply that term to "form." The form appears upon the common matter and "specifies" it, and makes it what it is, bringing it from potentiality to actuality. It is also the efficient and final cause of the body. The body exists for the sake of the soul, in order that the soul may attain its perfection through the body. As the most perfect body in the lower world is the human body, and it is for the sake of the soul, it follows that the existence of the sublunar world is for the sake of the human soul, that it may be purified and made perfect by science and moral conduct.