"The first and more manifest way is the argument from motion."Now, caution must be exercised here with the word "motion." He uses words differently from how we use them today. What he is talking about here is not necessarily motion from place to place, but rather something more like "change." Specifically, he is talking about the actualization of a potential. Or to put it another way, the coming to be of an object or attribute of an object that has the ability to be. The banana that is turning black on top of your fridge would be considered "motion" in the sense Aquinas means here.
"It is certain, and evident to our senses, that in the world some things are in motion."This is the empirical premise. Just look around you. Is there "motion" as meant above? Is the banana turning from yellow to black? Are trees outside swaying with the wind? Birds flying? Cars driving? People walking? In short, are there things becoming? Becoming older, becoming bigger, smaller. Becoming pieces. Becoming whole. Change. This is evident to our senses, and we can see it all around us all the time.
"Now whatever is in motion is put in motion by another..."Again we must be cautious. Let's rephrase it in modern terms: "whatever is changing is being changed by something else." But even more caution is warranted. What he means by "another" is not "something else" per se, but rather something that already exists. This could be a part of the thing that is changing, even an internal part. You may protest that you move yourself when you walk. That you are not moved by "another" at all. But think about what is being changed, here. Your upper body is being moved by your legs. Your upper body is not moving itself. It is being moved by something else. Something else that exists. If your legs didn't exist, they couldn't very well move your upper body.
"...for nothing can be in motion except it is in potentiality to that towards which it is in motion."Another tricky term! The word "potentiality" could perhaps best be defined as "ability to be." An acorn has the ability to be an oak tree. It may be eaten by a squirrel or dry up, but as an acorn, it has the ability to be an oak tree. A yellow banana has the ability to be black, whether it actually ever turns black or not because you eat it. So change is only possible because something has the ability to be whatever it is becoming. If it doesn't have the ability to be whatever it is becoming, then it just wouldn't be changing into that thing.
"...whereas a thing moves inasmuch as it is in act."Something can only cause change if it exists. If something doesn't exist, it can't cause change. Your upper body cannot be moved by non-existent legs. If you have no legs, either your upper body won't move, or something else is going to have to move it. If something is moving it, whatever that thing is must be something that actually exists.
"For motion is nothing else than the reduction of something from potentiality to actuality."Remember that "motion" means "change," and that "potentiality" means "ability to be." So this could be translated as "change is when something actually becomes what it has the ability to be." The acorn has the ability to be an oak tree, and when it actually become that oak tree, a change has occurred. This is all pretty basic stuff once the terminology is understood, isn't it?
"But nothing can be reduced from potentiality to actuality, except by something in a state of actuality."This is the same principle that was highlighted above. A change in something can only be caused by something else that already exists. The change in your upper body can only be caused by existent legs, or an existent wheel chair. The change in color in a banana can only be caused by existent enzymes, not non-existent enzymes.
"Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it."Here he gives an example, as I have already done a few times. Wood is potentially on fire, and if something comes along to set it on fire the wood becomes actually on fire. The potential becomes actual. One thing to note is that his example here might not be the best, since it gives the impression that whatever causes change in something must be in the same state, as when he says that something which is actually hot causes the wood to be actually hot. But this is not always the case. As I showed above enzymes cause bananas to turn black but may not actually be black themselves. They only have the ability to cause blackness in the banana.
"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."The banana cannot be both actually black, and potentially black (that is, yellow with the ability to be black) at the same time in the same respect (color). For then the banana would be both black, and not black, which is a logical contradiction.
"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."Therefore, something (a single part) cannot be both the initiator of change and thing being changed, with respect to the same property, such as color. Another slightly different way of making this point is that a potential (a property that has the ability to be but does not yet exist) cannot make itself real, because a potential doesn't even exist yet and therefore cannot very well cause anything, such as bringing itself into existence.
"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."Remember above, with the legs? The legs are moving your torso, but since they too are changing, they must be being changed by something else. Your legs are being moved by muscles. But the muscles are changing! And whatever is changing is being changed by something else. They are being changed by motorneurons. But wait! The motorneurons are changing! And so on. We have here a concurrent chain of changers. and this is vital to understand for the next point:
"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."Now, here is where most people REALLY mess up, and is by far the trickiest part of the argument. They see the premise that this cannot go on to infinity, and they usually imagine a chain stretching back in time to the first event, like the Big Bang. But Aquinas explicitly allows this type of series to stretch on indefinitely. Elsewhere, he says that an artificer may act by means of an infinite number of hammers, as one after another may be broken. There is nothing in this series of broken hammers that demands there be a first.
But consider a different kind of chain. A concurrent chain. A chain where an effect is being transmitted from one end to another. For example, a power plant is generating electricity, which is being transmitted through power lines to the lamp in your living room. If there is no power plant (or other source of electricity), then there is no electricity and your lamp won't be lit. If there is an infinite string of power lines, then there is by definition no power plant and your lamp won't be lit. But your lamp is lit, so there must be a source of electricity, so there cannot be an infinite string of power lines.
There are a few different ways to phrase this principle. Here is one: for any given effect, there must be some cause capable of producing it. The lamp being lit is the effect, and there must be some cause capable of producing that effect. But the power outlet is not capable of producing that effect, and neither is the power line by itself, and neither is an infinite number of power lines. An infinite number of powerless wires is just as powerless as a single set of wires.
This principle is often spoken of as a "vertical" causal series, moving down into the most fundamental aspects of reality in the present, as opposed to a "horizontal" causal series, moving back to earlier and earlier events.
Another way of understanding this point is to consider the cause of something becoming, vs the cause of something being. The cause of a lake's becoming would be the evaporation of water, which turns into clouds, then rain, which falls, collects in ground water, and becomes the lake. A series of events: event 1, then event 2, then event 3, and so on. But once a lake is in existence, it continues to exist from moment to moment due to factors other than the lake itself, and that is what Aquinas is getting at. The lake continues to exist from moment to moment because of warm air that keeps it from freezing, the warm air continues to exist because of the heat from the Sun, the heat from the Sun continues to exist from moment to moment because of nuclear reactions in the core, and so on. Notice how with this type of chain, we are moving "down" so to speak to more fundamental aspects of reality in the present, not in a chain of events stretching back in time.
You could think of each member of this chain as getting its realness, its existence, its actuality, from the next member in the chain, which in turns gets its actuality from the next member in the chain, and so on. And how I showed with the lamp and power plant, if there is no source of actuality (source of existence, or source of realness), then nothing would exist. But something does exist. So there is a source of existence, or source of actuality. Something that can cause existence without needing to be caused to exist. Or to put it another way, something that can actualize without needing to be actualized. And that is what Aquinas concludes next, in his own words:
"Therefore it is necessary to arrive at a first mover, put in motion by no other..."Note that the word "first" here does not mean first event, but rather it means something more like "source." Or "primary cause, rather than secondary cause." This follows from what I said above regarding a source of an effect. The Moon is the cause of light, but it is only a secondary cause, as it depends on the Sun for its light. The Sun is the source of light, and therefore "first" in the sense meant by Aquinas. And even if the Sun were infinitely old, it would still be the "first" cause of light, with the Moon the "second" cause of light.
"...and this everyone understands to be God."Wait, wait, wait! What?! This everyone understands to be God?! Aquinas has not proven this thing to be intelligent, or good, or anything else. What's he talking about?
Well, for one thing, he is writing to other Catholic priests, not to atheists, so he is not really building a case for an unbeliever to read raw. For another thing, he retroactively justifies the use of the term "God" in later arguments, when he shows the unmoved mover to be intelligent, good, and all the rest. So you might accuse him of jumping the gun a bit, but it's only semantics at this point. Just a label. In the next article, he goes on to ask, "Is God a body?", showing that he is just slapping a label on the unmoved mover even though we don't know what it's like yet. What Aquinas has at this point is simply something that is unchangeable but that causes other things to change, and that's it.