 The prevalent medieval conception of the sphere of the cosmos is that it is not a vacuum, but rather, crystalline and composed of a fifth element superior to the four elements of our terrestrial world. Hence, it would appear that the efficient spiritual cause could also be an efficient material cause. However, this difficulty is resolved if what Israeli meant here was an intelligence or soul governing the sphere. Although Israeli does not in any of his surviving writings and fragments mention a soul of the spheres, his pupil Dunash Ibn Tamim affirms it in his commentary on Sefer Yetzirah.