  An essentially "complete" description of the low-energy nuclear structure of
the superfluid nucleus $^{120}$Sn and of its odd-$A$ neighbors is provided by
the observations carried out with the help of Coulomb excitation and of one--
and of two-- particle transfer reactions, specific probes of vibrations,
quasiparticle and pairing degrees of freedom respectively, and of their mutual
couplings. These experimental findings are used to stringently test the
predictions of a similarly "complete" description of $^{119,120,121}$Sn carried
out in terms of elementary modes of excitation which, through their
interweaving, melt together into effective fields, each displaying properties
reflecting that of all others, there individuality resulting from the actual
relative importance of each one. Its implementation is done by solving the
Nambu-Gor'kov equations including, for the first time, all medium polarization
effects resulting from the interweaving of quasiparticles, spin and surface
vibrations, taking into account, within the framework of nuclear field theory
(NFT), the variety of processes leading to self-energy, vertex and Pauli
principle corrections, and to the induced pairing interaction. Theory provides
an overall quantitative account of the experimental findings. From these
results one can, not only obtain strong circumstantial evidence for the
inevitability for the dual origin of pairing in nuclei but also, extract
information which can be used at profit to quantitatively disentangle the
contributions to pairing correlations in general and to the pairing gap in
particular, arising from the bare and from the induced pairing interactions.
