  We prove that a Radon measure $\mu$ on $\mathbb{R}^n$ can be written as
$\mu=\sum_{i=0}^n\mu_i$, where each of the $\mu_i$ is an $i$-dimensional
rectifiable measure if and only if for every Lipschitz function
$f:\mathbb{R}^n\to\mathbb{R}$ and every $\varepsilon>0$ there exists a function
$g$ of class $C^1$ such that $\mu(\{x\in\mathbb{R}^n:g(x)\neq
f(x)\})<\varepsilon$.
