Full Idea
In Richard's puzzle, we use the notion of 'definition', and this, oddly enough, is not definable, and is indeed not a definite notion at all.
Gist of Idea
Richard's puzzle uses the notion of 'definition' - but that cannot be defined
Source
Bertrand Russell (On 'Insolubilia' and their solution [1906], p.209)
Book Reference
Russell,Bertrand: 'Essays in Analysis', ed/tr. Lackey,Douglas [George Braziller 1973], p.209
A Reaction
The background for this claim is his type theory, which renders certain forms of circular reference meaningless.