# The set of all sets that is not an element of a set does not exist

Assume the following set:

$\{X|X\notin A\}$

That is the set of all sets that is not an element of the set $A$. How can I prove this set does not exist?

Since the set $\mathcal{P}(A)$ of all subsets of $A$ exists, if your set existed, then their union would exist, too. But this union is the set of all sets, which doesn't exist.