The substances perceived by us are atoms which come into existence from vacuity and drop out of existence again. Thus, when a body moves from one position to another the atoms in the first position cease to be, and a group of new similar atoms come into existence in the second position, so that movement involves a series of annihilations and creations. The cause of these changes is God, the only permanent and absolute reality. There is no secondary cause, as there are no laws of nature; in every case God acts directly upon each atom. Thus, fire does not cause burning, but God creates a being burned when fire touches a body, and the burning is directly His work. So in the freedom of the will, as, for example, when a man writes, God gives the will to write and causes the apparent motion of the pen and of the hand, and also directly creates the writing which seems to proceed from the pen. Existence is the very self of the thing. This is peculiar to al-Ash`ari and his followers: all others hold existence to be the state (_hāl_) necessary to the essence, but in al-Ash`ari it is the essence. So God exists, and His existence is the self (_`ayn_) of His essence. Such a system involves ethical difficulties; it appears that there can be no responsibility if there is no connection between action and the act done. Al-Ash`ari replied that there is a unity in the will of God, so that cause and effect are not isolated as though independent atoms, but all is disposed according to a Divine plan. This answer, however, can hardly be regarded as adequate. This system is an attempt to deal with the difficulties raised by philosophy, but al-Ash`ari considers it preferable that the difficulties should never be raised, and so strongly urges that the mysteries of philosophy should never be discussed with the multitude. We shall see the same conclusion set forth by the later philosopher of the West, but on a somewhat different ground; they regarded the mysteries of philosophy as containing the supreme truth, for which the multitude was not ripe, and so they should not be discussed publicly, as the people were not able to understand; but al-Ash`ari seems rather to regard these mysteries as likely to be not edifying, as introducing questions which are of small importance compared with the great truths of revelation. The Ash`arite system thus described was completed by al-Baqilani (d. 403), but it did not become general until it was popularised by al-Ghazali in the East and by Ibn Tumart in the West.