Full Idea
In the family of the Liar is the Knower Paradox, where φ(x) is 'x is known to be true', and there is a set of known things, Kn. By knowing a sentence is not in the known sentences, you know its truth.
Gist of Idea
If you know that a sentence is not one of the known sentences, you know its truth
Source
Graham Priest (The Structure of Paradoxes of Self-Reference [1994], §4)
Book Reference
-: 'Mind' [-], p.30
A Reaction
[mostly my wording]