Equivalence of Definitions of Finer Topology

Theorem
Let $S$ be a set.

Let $\tau_1$ and $\tau_2$ be topologies on $S$.

Proof
Let $I_S: \struct {S, \tau_1} \to \struct {S, \tau_2}$ be the identity mapping on $S$.

Then:

$\quad \; \leadstoandfrom \: \: I_S$ is continuous, by definition of a continuous mapping, which is.