  The observed hierarchy in the fermion masses, which imply a set of small mass
ratios, is not naturally small regarding 't Hooft's criteria. In this work, in
a model independent approach, we introduce a set of conditions by which fermion
mass ratios become natural. Interestingly, these conditions demand that fermion
mixing should be described by the four independent mass ratios of each fermion
sector. Application of this set of conditions to the standard theory enables
one to understand the mains aspects in quark and lepton mixing. This feature
can be taken as a strong evidence for the existence in Nature of a flavour
symmetry. Also, for this analysis to work in the lepton sector, neutrino masses
should have normal ordering with the lightest neutrino mass satisfying the
lower bound, $m_{\nu 1} \geqslant (5.0\pm 0.1) \text{ meV}$, making the
approach testable.
