Set theory was created around 1874 by Georg Cantor. It had to be made better because collections of objects can cause problems if we work with them without explaining them better. Russell's paradox was one of the problems. Think about the set of all sets that are not members of themselves. If it were inside itself, then the rule which defines it would mean that it is "not" inside itself. But, if it were not inside itself, then the rule which defines it would mean that it "is" inside itself. This was a serious problem, and it meant that the old set theory was broken.