| context | $X,Y\in\mathfrak U$ |
| definiendum | $ x\in X \cap Y $ |
| postulate | $ x\in X \cap Y \Leftrightarrow (x\in X\land x\in Y) $ |
$ X \cap Y $ is commutative and idempotent.
The intersection and union are associative and distributive with respect to another.
Wikipedia: Intersection