Why if $f\in A[X]$ is reducible there is $f,g\in A[X]$ that are not invertible such that $f=gh$.

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Let $A$ a factorial ring. Why if $f\in A[X]$ is reducible there is $f,g\in A[X]$ that are not invertible such that $f=gh$. I don't understand in what $f,g$ can't be irreducible.