Baer's Criterion

Theorem
Let $R$ be a ring with unity.

Let $M$ be a left $R$-module.

Then $M$ is injective the following condition holds:
 * For all left ideals $I$ of $R$ with inclusion map $\iota : I \to R$, and for all $R$-module homomorphisms $f : I \to M$, there exists an $R$-module homomorphism $\tilde f : R \to M$ such that:
 * $\tilde f \circ \iota = f$