Full Idea
Why does it follow from there being only one thing that it is unmoving, since, for example, water moves internally while remaining one?
Gist of Idea
There could be movement within one thing, as there is within water
Source
comment on Parmenides (fragments/reports [c.474 BCE]) by Aristotle - Physics 186a16
Book Reference
Aristotle: 'Physics', ed/tr. Waterfield,Robin [OUP 1996], p.13
A Reaction
One suspects that Parmenides wasn't used to critical questions like this, and would have sharpened up his theory if it had been subjected to criticism. How big was the One? Maybe Aristotle is the real father of philosophy.