Answer:It is certain and evident to our senses that some things are in motion. Whatever is in motion is moved by another, for a thing can be in motion only if it has a potentiality for that towards which it is being moved, while a thing moves insofar as it is actual. for motion means the reduction of something from a state of potentiality into a state of actuality. but a thing can be reduced from a state of potentiality into a state of actuality only by something already in a state of actuality. thus that which is actually hot such as fire, makes wood, which is potentially hot, to be actually hot and thereby moves and changes it. it is impossible for the same thing should be simultaneously in a state of actuality and potentiality from the same point of view, but only from different points of view. what is actually hot cannot simultaneously be only potentially hot...if that by which is put in motion is itself put in motion, then this also must be put in motion by another, and that by another again. this cannot go on to infinity, because then there would be no first mover, and consequently, no other movers-since subsequent movers only move insofar as they are put int motion by the first mover, just as the staff only moves because it is put in motion by the hand. therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.
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