It must be something so great, so powerful that it can move on its self.
This Prime Mover must be God
Aquinas uses the word motion in to different ways:
M O T I O N
When something changes to one state. It moves from being potentially to (what it could be) to actually (actually being that thing)
E.g. wood is potentially hot. To become actually hot it must come into contact with something already actually hot.
Therefore there is a change of activities stretching back into history, which must start with the actual being.
Summary of the Second Way- St Thomas Aquinas
This is the argument from Causation to a First Cause.
Everything in our universe has a cause.
Nothing just happens there always needs to be a cause and the result of a cause is an effect.
Nothing in the universe is capable of causing itself. So a cause of an effect must have excited before the cause existed itself.
This is a continuing chain and is known as a chain of causes, which is named Regression of Causation.
Regression of causation must go backwards forever (infinite regression) or if not that then there must have been a first cause.
An infinite regression goes against all experiences of the universes. So there must be a first cause.
This first causes cannot have been caused by something else as then it would have been a second cause and therefore not a first cause.
A first cause which is so grateful /powerful that it could cause itself.
This first cause is God.
The second way is from the nature of efficient cause. In the world of sensible things we find there is an order of efficient causes. There is no case known (neither is it, indeed possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible. Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, whether the intermediate cause be several, or one only. Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, or intermediate, cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God
The notion of cause and effect, means you cannot have the latter (effect) without the former (cause - here called an efficient cause, because it is the means of bringing another thing into existence, or causing something to change). For Aquinas (and Aristotle) there cannot be an endless regression of cause and effect, and as such there must be a first cause, which is God.
The phrase in bold (above) suggests that there would be nothing here, if there was not an original cause of everything. As such, this means that the world and the universe cannot be infinite (i.e. have always existed).