Let $R$ be an associative unital ring, and let $R\operatorname{-Mod}$ denote the category of unital left $R$-modules. For every left $R$-module $M$, there exist an injective left $R$-module $E$ and an injective $R$-[linear map](/page/Linear%20Map)
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\begin{align*}
\iota: M \to E.
\end{align*}
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Equivalently, the category $R\operatorname{-Mod}$ has enough injective objects.