The universal set seemed extremely natural and obvious, yet ultimately several paradoxes of set theory were traced to the assumption that it existed, which mathematicians now know is flawed.

• 全集似乎极其自然且显而易见，但是最终，人们从集合论的几个悖论追溯到全集存在的假设，数学家现在知道，这一假设是有瑕疵的。