Only a small portion has been saved of the original writings of these men. A short dissertation by Qosta ibn Luqa on the distinction between Soul and Spirit (πνεῦμα, ruh), preserved in a Latin translation, has been frequently mentioned and made use of. According to it, the Spirit is a subtle material, which from its seat in the left ventricle of the heart animates the human frame and brings about its movements and perceptions. The finer and clearer this Spirit is, the more rationally the man thinks and acts: there is but one opinion upon this point. It is more difficult, however, to predicate anything sure, and universally valid, of the Soul. The deliverances of the greatest philosophers occasionally differ, and occasionally contradict each other. In any case the Soul is incorporeal, for it adopts qualities, and, in fact, qualities of the most opposite nature at one and the same time. It is uncompounded and unchangeable, and it does not, like the Spirit, perish with the body. The Spirit only acts as an intermediary between the Soul and the Body, and it is in this way that it becomes a secondary cause of movement and perception. The statement which has just been given regarding the Soul is found in many of the later writers. But by slow degrees, as the Aristotelian philosophy thrusts Platonic opinions more and more into the background, another pair of opposites come into full view. Physicians alone continue to speak of the importance of the ‘ruh’ or Spirit of Life. Philosophers institute a comparison between Soul and Spirit or Reason (νοῦς, ʻaql). The Soul is now reduced to the domain of the perishable, and sometimes, in Gnostic fashion, even to the lower and evil realm of the desires. The rational Spirit,—as that which is highest, that which is imperishable in man—is exalted above the Soul. In this notice, however, we are anticipating history: let us return to our translators.