Full Idea
The guarantee of immunity from error in prioprioception is only as strong as the guarantee that proprioception only ever represents the proprioceiving agent.
Clarification
'Proprioception' is awareness of your own body (usually!)
Gist of Idea
Proprioception is only immune from error if you are certain that it represents the agent
Source
Cappelen,H/Dever,Josh (The Inessential Indexical [2013], 07.1)
Book Reference
Cappelen,H/Dever,J: 'The Inessential Indexical' [OUP 2013], p.132
A Reaction
This is part of an interesting and sustained attack on the idea that self-knowledge is immune from error. They are thinking of science-fictiony situations where I am wired up to experience your leg movement. My experiences usually track me, that's all.