 The commentary on "de Caelo" was written before that on the "Physica Auscultatio", and probably not in Alexandria, since he mentions in it an astronomical observation made during his stay in that city by Ammonius.Simplicius wrote his commentary on the "Physica Auscultatio" after the death of Damascius, and therefore after his return from Persia.When it was that he wrote his explanations of the "Categories", whether before or after those on the above-mentioned Aristotelian treatises, it is impossible to ascertain.