  We show that B-type $\Pi$-stable D-branes do not in general reduce to the
(Gieseker-) stable holomorphic vector bundles used in mathematics to construct
moduli spaces. We show that solutions of the almost Hermitian Yang--Mills
equations for the non-linear deformations of Yang--Mills instantons that appear
in the low-energy geometric limit of strings exist iff they are $\pi$-stable, a
geometric large volume version of $\Pi$-stability. This shows that
$\pi$-stability is the correct physical stability concept. We speculate that
this string-canonical choice of stable objects, which is encoded in and derived
from the central charge of the string-\emph{algebra}, should find applications
to algebraic geometry where there is no canonical choice of stable
\emph{geometrical} objects.
