  If the quantum mechanical description of reality is not complete and a hidden
variable theory is possible, what arises is the problem to explain where the
rates of the outcomes of statistical experiments come from, as already noticed
by Land\'e and Popper. In this paper this problem is investigated, and a new
"paradigm" about the nature of dynamical and statistical laws is proposed. This
paradigm proposes some concepts which contrast with the usual intuitive view of
evolution and of physical law, such as: initial conditions could play no
privileged role in determining the evolution of the universe; the statistical
distribution of the particles emitted by a source could depend on the future
interactions of the particles; indeterministic trajectories can be defined by
the least action principle.
  This paradigm is applied to the analysis of the two-slit experiment and of
the EPR paradox, and a coherent picture for these phenomena is proposed; this
new picture shows how the well known difficulties in completing the quantum
mechanical description of reality could be overcome.
