Let $k$ be a field and $V$ a $k$-vector space. Then there is a map $V \to V^{\ast \ast}$, where $V^{\ast}$ is the dual vector space. If we are in ZFC and $\dim V$ is infinite, then this map is not surjective. As we learned in [this question](https://mathoverflow.net/questions/49351), there are models of $ZF$ where $V \to V^{\ast \ast}$ is an isomorphism when $V$ has a countable basis. I think the same argument shows that it is consistent with ZF that this is an isomorphism whenever $V$ has a basis.



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> Is it consistent with $ZF$ that $V \to V^{\ast \ast}$ is an isomorphism for all vector spaces $V$? 
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I ask because I'm teaching a rigorous undergrad analysis class. My students keep asking me whether they **have** to believe that $V \to V^{\ast \ast}$ can fail to be an isomorphism. Of course, I'm trying to change their intuition to point out why most mathematicians find the failure of isomorphism plausible and point out that there are more subtle ways to salvage the claim, such as Hilbert spaces, but I'd also love to be able to give them a choice free proof that there is some vector space where this issue comes up. 



Is it consistent with ZF that $V \to V^{\ast \ast}$ is always an isomorphism?
No, it’s not consistent.


Let $V=k^{(\omega)}$ be the vector space of finite sequences of elements of $k$. Then $V^\*$ can be identified with the vector space $k^\omega$ of all sequences, and elements of the image of the natural map $V\to V^{\*\*}$, considered as maps $k^\omega\to k$, are determined by their restriction to $k^{(\omega)}$.


So if $V\to V^{\*\*}$ is an isomorphism, then, taking $W=k^\omega/k^{(\omega)}$, there are no nonzero linear maps $W\to k$, and hence $W^{\*\*}=0$. But $W$ is nonzero.


So the map to the double dual must fail to be an isomorphism either for $V$ or for $W$.


