On Russel's Paradox
Russel's paradox is defining "the set of all sets that don't contain themselves". You can show that such construct leads to a contradiction. Read the wiki link for details. I started this article as answer to this article . There is another interesting paradox discussed by Bertrand Russell: "The smallest positive integer not definable in under sixty letters", actually better known as Berry's paradox You can also prove that such number creates a contradiction, because you just defined it under sixty letters. For me, the above paradox shows that it is not enough to create a condition for something to exist with that condition - or a set of such elements. Such paradoxes argue that such objects defined by a predicate cannot exist sometimes.