 Whereas the soul functions as the efficient cause of motion, the separate unmoved movers function as the final cause of motion insofar as they represent an object of love and desire. The point is that since celestial souls are corporeal and finite, they are insufficient to account for the spheres’ perpetual activities. The perpetuation of motion, accordingly, must be the function of immaterial and incorruptible principles. These are the unmoved separate intellects of which the mover of the outermost spheres is the highest in rank (Avicenna Shifāʾ [Metaphysics]: 308; cf. Maqāṣid: 149).