Let $H_{\mathbf{Q}}$ and $H_{\mathbf{Q}}^'$ be two pure Hodge structures of weight $n$ and $n'$ respectively. How do you prove the following simple fact:

fact: If $n>n'$ and $f:H_{\mathbf{Q}}\rightarrow H_{\mathbf{Q}}^'$ is a morphism which respects the filtrations over $\mathbf{C}$, then $f=0$.

I don't quite see how to use the assumption $n>n'$...