In this paper, we study the counting functions $M_\mathcal{P}(x)$,
$\psi_\mathcal{P}(x)$ and $N_\mathcal{P}(x)$ of a generalized prime system
$\mathcal{N}$. Here $M_\mathcal{P}(x)$ is the partial sum of the M\"{o}bius
function over $\mathcal{N}$ not exceeding $x$. In particular, we study these
when they are asymptotically well-behaved, in the sense that $N_{\cal{P}}(x) =
\rho x+O({x^{ \beta+\eps }})$, $\psi_{\cal{P}}(x) = x+O({x^{ \alpha+\eps }})$
and $ M_\mathcal{P}(x) = O(x^{\gamma+\eps})$, for some $\rho >0$ and $\alpha,
\beta, \gamma<1$. We show that the two largest of $\alpha,\beta,\gamma$ must be
equal and at least $\frac{1}{2}$.