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Let $f:X\to Y$ be a morphism in a category. It is easy to see that if $f$ is a monomorphism then there exists a pullback $X \times_Y X$.

Here the question is whether the converse is true.

If two projections of $X\times_Y X$ equal then it is easy to check.

So my question is rewritten as whether the two projections equal always.

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    No. He is right. I misunderstood what he means.2012-10-21

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In Set, the pullback $X \times_Y X$ always exists and is given by $\left\{ (x,x') \in X \times X \, \mid\, f(x) = f(x') \right\},$ regardless of whether $f$ is a monomorphism or not.

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The correct statement is that $f$ is a monomorphism if and only if the pullback $X \times_Y X$ not only exists but is naturally isomorphic to $X$.

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    "Naturally" here means that the two projections are equal and are isomorphisms.2012-10-21