Fourth Method.—The actual production of a thing is preceded in time by its possibility. The actual change of a thing is likewise preceded in time by its possibility. From this proposition Aristotle derives the eternity of the circular motion of the spheres. The Aristotelians in more recent time employ this proposition in demonstrating the Eternity of the Universe. They argue thus: When the Universe did not yet exist, its existence was either possible or necessary, or impossible. If it was necessary, the Universe could never have been non-existing; if impossible, the Universe could never have been in existence; if possible, the question arises, What was the substratum of that possibility? for there must be in existence something of which that possibility can be predicated. This is a forcible argument in favour of the Eternity of the Universe. Some of the later schools of the Mutakallemim imagined that they could confute this argument by objecting that the possibility rests with the agent, and not with the production. But this objection is of no force whatever; for there are two distinct possibilities, viz., the thing produced has had the possibility of being produced before this actually took place; and the agent has had the possibility of producing it before he actually did so. There are, therefore, undoubtedly two possibilities—that of the substance to receive a certain form, and that of the agent to perform a certain act. These are the principal methods, based on the properties of the Universe, by which Aristotle proves the Eternity of the Universe. There are, however, other methods of proving the Eternity of the Universe. They are based on the notions formed of God, and philosophers after Aristotle derived them from his philosophy. Some of them employed the following argument:— Fifth Method.—If God produced the Universe from nothing, He must have been a potential agent before He was an actual one, and must have passed from a state of potentiality into that of actuality—a process that is merely possible, and requires an agent for effecting it. This argument is likewise a source of great doubts, and every intelligent person must examine it in order to refute it and to expose its character. Sixth Method.—An agent is active at one time and inactive at another, according as favourable or unfavourable circumstances arise. The unfavourable circumstances cause the abandonment of an intended action. The favourable ones, on the other hand, even produce a desire for an action for which there has not been a desire previously. As, however, God is not subject to accidents which could bring about a change in His will, and is not affected by obstacles and hindrances that might appear or disappear, it is impossible, they argue, to imagine that God is active at one time and inactive at another. He is, on the contrary, always active in the same manner as He is always in actual existence.