Averroes also wrote stand-alone philosophical treatises, including On the Intellect, On the Syllogism, On Conjunction with the Active Intellect, On Time, On the Heavenly Sphere and On the Motion of the Sphere. He also wrote several polemics : Essay on al-Farabi 's Approach to Logic, as Compared to that of Aristotle, Metaphysical Questions Dealt with in the Book of Healing by Ibn Sina, and Rebuttal of Ibn Sina's Classification of Existing Entities.