The nature of the soul has been a puzzle to thinkers and philosophers from time immemorial. Some thought it was a material substance, some regarded it as spiritual. It was identified with the essence of number by the Pythagoreans. And there have not been wanting those who, arguing from its dependence upon body, said it was an accident and not a substance. Strange to say the Mutakallimun, defenders of religion and faith, held to this very opinion. But it is really no stranger than the maintenance of the soul's materiality equally defended by other religionists, like Tertullian for example, and the opposition to Maimonides's spiritualism on the part of Abraham ben David of Posquières. The Mutakallimun were led to their idea by the atomic theory, which they found it politic to adopt as more amenable to theological treatment than Aristotle's Matter and Form. It followed then according to some of them that the fundamental unit was the material atom which is without quality, and any power or activity in any atom or group of atoms is a direct creation of God, which must be re-created every moment in order to exist. This is the nature of accident, and it makes more manifest the ever present activity of God in the world. Thus the "substantial" or "accidental" character of the soul is one that is touched on by most Jewish writers on the subject. And Israeli also refers to the matter incidentally in the "Book of the Elements."[34] Like the other Jewish philosophers he defends its substantiality. The fact of its separability from the body, he says, is no proof of its being an accident. For it is not the separability of an accident from its substance that makes it an accident, but its destruction, when separated. Thus when a white substance turns green, the white color is not merely separated from its substance but ceases to exist. The soul is not destroyed when it leaves the body. Another argument to prove the soul a substance is this. If the soul were an accident it should be possible for it to pass from the animal body to something else, as blackness is found in the Ethiopian's skin, in ebony wood and in pitch. But the soul exists only in living beings. We find, besides, that the activity of the soul extends far beyond the body, and acts upon distant things without being destroyed. Hence it follows that the soul itself, the agent of the activity, keeps on existing without the body, and is a substance.