Let $\mathbb{N}$ be the D225: Set of natural numbers.
A D11: Set $X$ is a countable set if and only if
\begin{equation}
\exists \, E \subseteq \mathbb{N} :
\text{Bij}(E \to X)
\neq \emptyset
\end{equation}
| (1) | $E \subseteq \mathbb{N}$ is a D78: Subset of $\mathbb{N}$ |
| (2) | $f$ is a D468: Bijective map from $E$ to $X$ |