  We propose a conjecture that leads to the determination of the intrinsic
scale (the scale at which the coupling constant diverges) for any gauge theory
by comparison to another theory for which there is phenomenological
information, provided that the two theories are placed in the same space-time
volume. This conjecture is verified independently for $U(1)_{em}$ and for
$QCD$. In this framework and from this perspective we show that the group
$SO(10)$ is the only viable candidate among the supersymmetric GUT groups
$SU(5)$, $SO(10)$ and $E_6$.
