  The gauge approach to the theory of gravity has been widely discussed as an
alternative to standard general relativity. The Poincar{\'e} group, as a
symmetry group of all relativistic theories in the absence of gravitation,
constitutes the most natural candidate for a gauge group. Although the
Poincar{\'e} gauge theory of gravity has been elaborated over the years and
cast into a beautiful formal framework, some fundamental problems have remained
unsolved. One of them concerns the inclusion of matter. The minimal coupling
procedure, which is employed in standard Yang--Mills theories, appears to be
ambiguous in the case of gravity. We propose a slight modification of this
procedure, which removes the ambiguity. Our modification justifies some earlier
results concerning the consequences of the Poincar{\'e} gauge theory of
gravity. In particular, the predictions of Einstein--Cartan theory with
fermionic matter are rendered unique. We recall the earlier proposed solution
based on modified volume--forms. The advantage of our modification is that the
predictions of the theory are not radically changed. Basically, this
modification simply justifies the results that were obtained partly `by chance'
in the hitherto prevailing accounts on the Einstein--Cartan theory. The only
difference in the predictions, when compared to the standard treatment,
concerns the Proca field in the presence of gravity. The `torsion
singularities' which occur there are shifted towards other values of the field.
