I am looking for the proposition which states that a theory rules scope cannot defined all versions of a proposition because it is kind of creating a circle.
Forgive my french.. And approximation.
Could anybody point me in the direction.. Preferably not the door.
Edit : upon an old reading (more than 10 years ago, sorry I'm an old guy), I was trying to recall why a theory needed external rules to be complete or valid and couldn't hold all the terms to its own validation/invalidation. @Dave pointed me on the right track, even if I'd like to expand this concept to a more generic one, philosophically speaking.
Now, I really don't know (and humbly speaking) how to precise something that I don't know : I started the readings that @Dave gave me, maybe I will be more able to narrow. Until then, I will gladly submit to your enlightened advice.
A good place to begin is Gödel's Proof by Ernest Nagel and James R. Newman (1958; freely available).
A bit more advanced is Gödel's Theorem by Torkel Franzén (2005; review).
(ADDED 9 DAYS LATER) The following book gives a super-gentle introduction to this general topic. I don't know how I forgot about this book --- it was all the rage towards the end of my undergraduate studies (I remember reading about, and sometimes even overhearing discussions about, courses being based on it), it won both the Pulitzer Prize for general non-fiction and the [USA] National Book Award for Science, and it launched the career of Hofstadter as one of the few mathematicians/physicists (his Ph.D. was in physics) whose celebrity has reached the general public.
Gödel, Escher, Bach: An Eternal Golden Braid by Douglas R. Hofstadter (originally published in 1979)