It is an infallibly defined article of Catholic faith that the existence of God is rationally demonstrable.
St. Paul the Apostle says, “For the invisible things of [God], from the creation of the world, are clearly seen, being understood by the things that are made; his eternal power also, and divinity.” (Romans 1:20)
Meanwhile, the First Vatican Council declared, “If anyone shall have said that the one true God, our Creator and our Lord, cannot be known with certitude by those things which have been made, by the natural light of human reason: let him be anathema.”
Hence, Aquinas presents five proofs for God’s existence in the Summa Theologica, which are often called “the five ways”. He also offers others proofs in other texts, such as De Ente et Essentia and the Summa Contra Gentiles.
The First Way – the argument from motion
This argument uses the word “motion” in the broad, Aristotelian sense of “change”. A rock falling to the earth, water freezing, a plant growing and a child being conceived are all cases of “motion”.The argument speaks of “potentiality” and “actuality”. For Aquinas, again following Aristotle, all physical beings are composed of potentiality and actuality (matter and form). Potentiality means potentiality for something – a rock in mid-air is potentially on the ground, water is potentially ice etc. Motion is the actualization (becoming actual) of something that is initially only a potentiality.
“The first and more manifest way is the argument from motion. It is certain, and evident to our senses, that in the world some things are in motion. Now whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves [i.e. makes something else move] inasmuch as it is in act. For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it. Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself. Therefore, whatever is in motion must be put in motion by another.”
Note here that, for Aquinas, when one thing puts another thing in motion, the action of “putting in motion” is simultaneous with the motion itself. (In fact, Aquinas thinks that this action in one sense is the motion.)
“If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again.”
Because moving and being-moved are simultaneous, we must not imagine that Aquinas here has in mind a regress of movers back in time. All of the movers in this regress exist, and act, simultaneously.
“But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand.”
The example of the hand moving the staff underscores the fact that the regress of movers does not go back in time. Aquinas is not saying that, millions of years ago, there had to be some first mover who “kick-started” all subsequent motions. In fact, he does not believe that an infinite regress back in time is necessarily impossible.
“Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.”
It isn’t immediately obvious that Aquinas has proved the existence of only one Unmoved Mover.