Full Idea
The Achilles argument is that (if the front runner keeps running) each time the pursuer reaches a spot where the pursuer has been, the pursued has moved a bit beyond.
Gist of Idea
Whenever the pursuer reaches the spot where the pursuer has been, the pursued has moved on
Source
Willard Quine (The Ways of Paradox [1961], p.03)
Book Reference
Quine,Willard: 'Ways of Paradox and other essays' [Harvard 1976], p.3
A Reaction
Quine is always wonderfully lucid, and this is the clearest simple statement of the paradox.