A flow $f$ is maximum $\Leftrightarrow$ $\exists$ an empty $s$-$t$ cut

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Given a flow network $(G,\mu,s,t)$. I am thinking about the following statement.
A flow $f$ is maximum $\Leftrightarrow$ $\exists$ an empty $s$-$t$ cut
I know "$\Rightarrow$ is correct, but also "$\Leftarrow$"?