Empty Set is Open in Metric Space

Theorem
Let $M = \left({A, d}\right)$ be a metric space.

Then the empty set $\varnothing$ is an open set of $M$.

Proof
By definition, an open set $S \subseteq A$ is one where every point inside it is an element of an open ball contained entirely within that set.

That is, there are no points in $S$ which have an open ball some of whose elements are not in $S$.

As there are no elements in $\varnothing$, the result follows vacuously.