  We analyze two-particle renormalizations within many-fermion perturbation
expansion. We show that present diagrammatic theories suffer from lack of a
direct diagrammatic control over the physical two-particle functions. To
rectify this we introduce and prove a Ward identity enabling an explicit
construction of the self-energy from a given two-particle irreducible vertex.
Approximations constructed in this way are causal, obey conservation laws and
offer an explicit diagrammatic control of singularities in dynamical
two-particle functions.
