The obscurity of this statement has led to a great divergence in its treatment by commentators. Theophrastus offers cautious suggestions and evidently regards the rational soul as differing only in degree of evolution from the lower forms of soul faculty. It was Alexander of Aphrodisias who opened up new fields of speculation, distinguishing between a material intellect and an active intellect. The former is a faculty of the individual soul and this it is which is the form of the body, but it means no more than the capacity for thinking and is of the same source as the other faculties of the human soul. The active intellect is not a part of the soul but is a power which enters it from outside and arouses the material intellect to activity; it is not only different in source from the material soul, but different in character in that it is eternal and so always has been and always will be, its rational power existing quite apart from the soul in which the thinking takes place; there is but one such substance and this must be identified with the deity who is the First Cause of all motion and activity, so that the active intellect is pictured as an emanation from the deity entering the human soul, arousing it to the exercise of its higher functions, and then returning to its divine source. This theistic interpretation of Aristotle was strongly opposed by the commentator Themistius who considers that Alexander forces the statement of the text out of its natural meaning and draws an unwarrantable deduction from the two sentences “these differences must be present in the soul,” and “this alone is immortal and eternal.” It seems, however, that Alexander’s interpretation played an important part in the formation of neo-Platonic theory, and it certainly is the key to the history of Muslim philosophy, and is not without its importance in the development of Christian mysticism.