Does the set of all sets contain itself? Why or why not?
There cannot be a set of all sets U that contains itself in standard set theory. One argument against it is Cantors theorem: If you take the power set P(U) of the mentioned set U, it has to have strictly larger cardinality than U. This implies that the power set P(U) is not contained in U, thus U cannot contain all sets.
But given some more restrictive axioms one can create a set of all valid sets that does include itself.
In what sens can we speak of valid sets?