Infimum is Unique

Theorem

Let $\struct {S, \preceq}$ be an ordered set.

Let $T$ be a non-empty subset of $S$.

Then $T$ has at most one infimum in $S$.

Let $c$ and $c'$ both be infima of $T$ in $S$.

From the definition of infimum, $c$ and $c'$ are lower bounds of $T$ in $S$.

By that definition:

$c$ is a lower bound of $T$ in $S$ and $c'$ is an infimum of $T$ in $S$ implies that $c \preceq c'$

$c'$ is a lower bound of $T$ in $S$ and $c$ is an infimum of $T$ in $S$ implies that $c' \preceq c$.

So:

$c' \preceq c \land c \preceq c'$

and thus by the antisymmetry of the ordering $\preceq$:

$c = c'$

$\blacksquare$