  The chiral susceptibility is given by the scalar vacuum polarisation at zero
total momentum. This follows directly from the expression for the vacuum quark
condensate so long as a nonperturbative symmetry preserving truncation scheme
is employed. For QCD in-vacuum the susceptibility can rigorously be defined via
a Pauli-Villars regularisation procedure. Owing to the scalar Ward identity,
irrespective of the form or Ansatz for the kernel of the gap equation, the
consistent scalar vertex at zero total momentum can automatically be obtained
and hence the consistent susceptibility. This enables calculation of the chiral
susceptibility for markedly different vertex Ansaetze. For the two cases
considered, the results were consistent and the minor quantitative differences
easily understood. The susceptibility can be used to demarcate the domain of
coupling strength within a theory upon which chiral symmetry is dynamically
broken. Degenerate massless scalar and pseudoscalar bound-states appear at the
critical coupling for dynamical chiral symmetry breaking.
