Intersection is Subset/General Result

Theorem

Let $S$ be a set.

Let $\mathcal P \left({S}\right)$ be the power set of $S$.

Let $\mathbb S \subseteq \mathcal P \left({S}\right)$.

Then:

$\displaystyle \forall T \in \mathbb S: \bigcap \mathbb S \subseteq T$

In the context of a family of sets, the result can be presented as follows:

Let $\left \langle{S_\alpha}\right \rangle_{\alpha \mathop \in I}$ be a family of sets indexed by $I$.

Then:

$\displaystyle \forall \beta \in I: \bigcap_{\alpha \mathop \in I} S_\alpha \subseteq S_\beta$

$\displaystyle x$

$\in$

$\displaystyle \bigcap \mathbb S$

$\quad$

$\displaystyle \implies \ \ $

$\, \displaystyle \forall T \in \mathbb S: \, $

$\displaystyle x$

$\in$

$\displaystyle T$

$\quad$ Definition of Set Intersection

$\quad$

$\displaystyle \implies \ \ $

$\, \displaystyle \forall T \in \mathbb S: \, $

$\displaystyle \bigcap \mathbb S$

$\subseteq$

$\displaystyle T$

$\quad$ Definition of Subset

$\quad$

$\blacksquare$