  A corpuscular simulation model for second-order intensity interference
phenomena is discussed. It is shown that both the visibility ${\cal V}=1/2$
predicted for two-photon interference experiments with two independent sources
and the visibility ${\cal V}=1$ predicted for two-photon interference
experiments with a parametric down-conversion source can be explained in terms
of a locally causal, modular, adaptive, corpuscular, classical
(non-Hamiltonian) dynamical system. Hence, there is no need to invoke quantum
theory to explain the so-called nonclassical effects in the interference of
signal and idler photons in parametric-down conversion. A revision of the
commonly accepted criterion of the nonclassical nature of light is needed.
