A set can totally contain itself. A better question would be: Consider a set, that contains all sets, that do not contain themself. Would that set contain itself?
Yes, just relax the axiom of comprehension, allow U ∈ U and move on with proving things for fun and profit. No one said that you have to pick axioms that seem natural or intuitive.
A set can totally contain itself. A better question would be: Consider a set, that contains all sets, that do not contain themself. Would that set contain itself?
It would. Source: I just shaved my beard
Yes, just relax the axiom of comprehension, allow U ∈ U and move on with proving things for fun and profit. No one said that you have to pick axioms that seem natural or intuitive.