An isomorphism of Hilbert spaces, $H \xrightarrow{\varphi} G$, is a linear mapping that is one-to-one and onto, and with $\left(\varphi(h), \varphi\left(h^{\prime}\right)\right)=\left(h, h^{\prime}\right)$. Prove that the free Hilbert spaces on sets $K$ and $K^{\prime}$ are isomorphic if and only if these sets are isomorphic.