  We show that doped Mott insulators exhibit a collective degree of freedom,
not made out of the elemental excitations, because the number of
single-particle addition states at low energy per electron per spin is greater
than one. The presence of such a collective degree of freedom which is not a
consequence of proximity to a phase transition is a consequence of dynamical
spectral weight transfer from high to low energies. This physics is captured by
the charge $2e$ boson that emerges by explicitly integrating out the
high-energy scale in the Hubbard model. The charge $2e$ boson binds to a hole,
thereby mediating new charge $e$ states at low energy. It is the presence of
such charge $e$ states which have no counterpart in the non-interacting system
that provides the general mechanism for the breakdown of Fermi liquid theory in
doped Mott insulators. The relationship between the charge $2e$ boson
formulation and the standard perturbative treatment is explained.
