As the title indicates, the anonymous treatise is concerned primarily with the nature of the soul. Whatever other topics are found therein are introduced for the bearing they have on the central problem. A study of the soul means psychology as well as ethics, for a complete determination of the nature of the soul necessarily must throw light not only upon the origin and activity of the soul, but also upon its purpose and destiny. The first error, we are told, that we must remove concerning the soul, is the doctrine of the "naturalists," with whom the Muʿtazilites among the Arabs and the Karaites among the Jews are in agreement, that the soul is not an independent and self-subsistent entity, but only an "accident" of the body. Their view is that as the soul is a corporeal quality it is dependent for its existence upon the body and disappears with the latter. Those of the Muʿtazilites who believe in "Mahad" (return of the soul to its origin), hold that at the time of the resurrection God will bring the parts of the body together with its accident, the soul, and will reward and punish them. But the resurrection is a distinct problem, and has nothing to do with the nature of the soul and its qualities. The true opinion, which is that of the Bible and the true philosophers, is that the soul is a spiritual substance independent of the body; that it existed before the body and will continue to exist after the dissolution of the latter. The existence of a spiritual substance is proved from the presence of such qualities as knowledge and ignorance. These are opposed to each other, and cannot be the qualities of body as such, for body cannot contain two opposite forms at the same time. Moreover, the substance, whatever it be, which bears the attributes of knowledge and ignorance, can receive them without limit. The more knowledge a person has, the more capable he is of acquiring more. No corporeal substance behaves in this way. There is always a limit to a body's power of receiving a given accident. We legitimately conclude, therefore, that the substance which bears the attributes of knowledge and ignorance is not corporeal but spiritual.[141] To understand the position of the soul and its relation to the body, we must have an idea of the structure and origin of the universe. The entire world, upper as well as lower, is divided into two parts, simple and composite. The simple essences, which are pure and bright, are nearer to their Creator than the less simple substances which come after. There are ten such creations with varying simplicity, following each other in order according to the arrangement dictated by God's wisdom. As numbers are simple up to ten, and then they begin to be compound, so in the universe the ten simple substances are followed by composite.