  We propose a non-perturbative calculation scheme which is based on the
semi-classical approximation of QCD and can be used to evaluate quantities of
interest in hadronic physics. As a first application, we evaluate the mass of
the pion and of the nucleon. Such masses are related to a particular
combination of Green functions which, in some limit, is dominated by the
contribution of \emph{very small-sized} instantons. The size distribution of
these pseudo-particles is determined by the 't Hooft tunneling amplitude
formula and therefore our calculation is free from any model parameters. We
prove that instanton forces generate a light pion and a nucleon with realistic
mass ($M_n \sim 970 MeV$). In connection with sum-rules approaches, we discuss
the overlap of instantons with pion and nucleon resonances.
