"Perpetual Motion" is a science fiction short story by American writer L. Sprague de Camp, part of his Viagens Interplanetarias series.

All is going as planned until the knight Shurgez, former paramour of Zerdai, a female member of the order Borel has taken up with, returns from a quest and challenges him to a duel over her.

Borel pretends to agree, but knowing himself no match for a trained warrior prepares for a quick getaway, which he effects on the very occasion of the duel itself.

Fleeing through the Koloft Swamp on a swift aya with as much of his ill-gotten gains as he could stow, he is attacked by the tailed aborigines dwelling there and forced to abandon his treasure to save his life.

As Novorecife has no extradition treaty with Mikardand, the authorities are forced to let him go, and soon Borel is working a new con on a visiting Viagens Interplanetarias bigwig, who is interested in touring the native kingdoms and is looking for a guide... As Judge Keshavachandra ruefully noted after the conclusion of Borel's trial: "Talk of perpetual motion, he's it!"