As best I can tell the arguments in chapter 13 are really three. One. Motion is imparted from one body to another. Two. Motion must start somewhere. Three. All of these weird things in the world obey the same laws.
These three tell Aquinas that there must be an origin of this.
Note that he grants almost no characteristics or properties to the unmoved mover at this point.
I think this is the point where acknowledgment of laws of motion and an abhorrence of paradox should bring almost anyone to be a deist.
