(related to Corollary: Uniqueness of the Empty Set)

- For any two given non-empty sets $X,Y$ assume, their elements $x\in X,y\in Y$ are sets themselves.
- By the extensionality principle they are equal $x=y$ if they have the same elements.
- This argument can be repeated for the elements of these elements (if any). We can repeat this argument for all elements of elements of elements, etc.
- At some point, we might arrive at elements $x\in X$ and $y\in Y$ which do not contain any elements. Note that the existence of such empty elements $x=\emptyset$ and $y=\emptyset$ is ensured by the axiom of empty set.
- Now, the extensionality principle says again, that $x=y$ are equal, even though $x=\emptyset$ and $y=\emptyset$.
- In other words, all empty sets are not distinguishable. Thus, $\emptyset$ is unique.∎