  We demonstrate how to identify which physical processes dominate the
low-energy spectral functions of correlated electron systems. We obtain an
unambiguous classification through an analysis of the equation of motion for
the electron self-energy in its charge, spin and particle-particle
representations. Our procedure is then employed to clarify the controversial
physics responsible for the appearance of the pseudogap in correlated systems.
We illustrate our method by examining the attractive and repulsive Hubbard
model in two-dimensions. In the latter, spin fluctuations are identified as the
origin of the pseudogap, and we also explain why $d-$wave pairing fluctuations
play a marginal role in suppressing the low-energy spectral weight, independent
of their actual strength.
