A look at Aristotle's famous argument for an unmoved mover, which can be read in Metaphysics, Book XII, parts 6 to 8, and in Physics, Book VII.
I. The Universe is Infinitely Old
To begin with, Aristotle argues that change and time must be infinitely old, and hence the universe must have existed forever. This is because if a change occurs, something has to cause that change, but then that thing changed in order to cause the change so something must have caused it, and so on back into infinity:
II. Something Cannot Change Itself
He then argues that something cannot change itself. This is because the future state of something does not exist yet, and so cannot make itself real. Only something that already exists can cause a change to happen. So any change that is occurring must have some cause:
But the cold air is itself changeable as well. It causes the water to change into ice, but it itself can change by becoming warm, or changing location, etc. Call it a "changeable changer."
III. There Must Be an Unchangeable Changer
If everything were a changeable changer, then it would be possible for change to stop happening. Because changeable changers, by their very nature, could stop causing change, and so it is possible that there could be a gap, wherein everything stops changing:
But change cannot stop, as per the first argument Aristotle gives. It has been going eternally, and will never stop. So not everything is a changeable changer. There must be at least one UNchangeable changer. Or to use the classic terminology, an "unmoved mover." Something that causes change, without itself changing, which provides a smooth, continuous source of eternal change:
IV. Attributes of the Unmoved Mover
The unmoved mover must be immaterial, because matter is changeable.
The unmoved mover must cause change as an attraction, not as an impulsion, because it cannot itself change. In other words, as an object of desire. This way it can cause change (by attracting things to it) without itself changing.
As an object of desire, it must be intelligible.
As an intelligible being, it must also be intelligent.
As an intelligent being, it thinks about whatever is good, which is itself. So it thinks about itself.