  I present a theoretical discussion of the uncertainties related to the QCD
analysis of the proton structure function $F_2(x,Q^2)$ at small $x$. The role
played by the `unphysical' gluon density is pointed out. It is shown how the
study of more observables can reduce the theoretical uncertainty and, in
particular, an alternative method of analysis, based on the introduction of
physical anomalous dimensions, is suggested.
