The Immovable Mover or Unmoved Mover is the first cause of existence—that which existed prior to existence—the thing that started it all. If the law of causality is true the effect "existence" must have a cause. Since causes always precede effects there must have been something that existed prior to anything existing. But obviously nothing could have existed prior to the first existent, so there could not have been a “first cause” and therefore the effect—existence, can have no cause. This means that it is possible (since we know that existence exists) that at least one effect—existence does not need to have a cause. As you know the law of causality cannot be true and false at the same time so existence must have always existed and there must not ever had been a time prior to existence, i.e., complete nothingness never existed. Even if there was a God you would have to say that he is something, for if God is nothing then he would not exist and something that does not exist cannot possibly be the “first cause.” As you can see, the Immovable Mover is impossible so the influence of Aristotle’s theory regarding its reality was colossal. He proved the existence of something that logically cannot exist. For more information regarding Aristotle’s theory of the Immovable Mover check out this site:
http://www.abu.nb.ca/Courses/GrPhil/PhilRel/Aristotle.htm