knowledge fractal
The basic problem can easily be explained by the following image:
a hand sketches a hand which sketches this hand and so forth...
You are in a similar situation if you say: »I am lying.« Are you really lying at that moment or are you telling the truth? (The classical example in this context is the Liar Paradox of the Cretan, Epimenides, who maintains that all Cretans lie.)
An example by Russel, which is rather more difficult to understand, concerns the set R of all sets which do not contain themselves as an element. If R is not contained in itself, does R have to be contained in itself?