# Why did mathematicians take Russell’s paradox seriously?

Though I’ve understood the logic behind’s Russell’s paradox for long enough, I have to admit I’ve never really understood why mathematicians and mathematical historians thought it so important. Most of them mark its formulation as an epochal moment in mathematics, it seems to me. To my uneducated self, Russell’s paradox seems almost like a trick question, like a child asking what the largest organism on Earth is, and getting back an answer talking about a giant fungus.

Well, OK…”

The set of all sets admits all sorts of unmathematical objects right? Chickens, spoons, 2 $\times$ 4s, what have you…I can’t imagine a situation where a mathematician solving a concrete problem would invoke a set like that. The typical sets one sees in everyday mathematics are pretty well defined and limited in scope: $\mathbb{Z}$, $\mathbb{R}$, $\mathbb{C}$, $\mathbb{R}^n, \mathbb{Q}_p$, subsets of those things. They are all built off each other in a nice way.

But clearly important people think the result significant. It prompted Gottlob Frege, a smart and accomplished man, who Wikipedia tells me extended the study of logic in non-trivial ways, to say that the “foundations of [his] edifice were shaken.” So I suppose Russell’s paradox is “deep” somehow? Is there a practical example in mathematics that showcases this, where one might be led horribly astray if one doesn’t place certain restrictions on sets, in say, dealing with PDEs, elliptic curves, functional equations? Why isn’t Russell’s paradox just a “gotcha”?