Full Idea
The one doesn't change places by going somewhere and coming to be in something, not does it move by spinning in the same location or by being altered, …so the one is therefore unmoved by every sort of motion.
Gist of Idea
The one is completely unmoving, because no types of motion are possible for it
Source
Plato (Parmenides [c.366 BCE], 139a)
A Reaction
This is the culmination of a sequence of arguments. The argument is here proposed by a semi-fictional Parmenides long after his death. This is a sort of ontological argument, drawing inferences from the nature of the central concept.
Book Reference
Plato: 'Parmenides', ed/tr. Gill,M.L./Ryan,P. [Hackett 1996], p.143