  We consider a simple model system supporting stable solitons in two
dimensions. The system is the parametrically driven damped nonlinear
Schr\"odinger equation, and the soliton stabilises for sufficiently strong
damping. The purpose of this note is to elucidate the stabilisation mechanism;
we do this by reducing the partial differential equation to a
finite-dimensional dynamical system. Our conclusion is that the negative
feedback loop occurs via the enslaving of the soliton's phase, locked to the
driver, to its amplitude and width.
