Full Idea
Richard: φ(x) is 'x is a definable real number between 0 and 1' and ψ(x) is 'x is definable'. We can define a real by diagonalization so that it is not in x. It is and isn't in the set of reals.
Gist of Idea
By diagonalization we can define a real number that isn't in the definable set of reals
Source
Graham Priest (The Structure of Paradoxes of Self-Reference [1994], §3)
Book Reference
-: 'Mind' [-], p.29
A Reaction
[this isn't fully clear here because it is compressed]