The suppression effect observed in masking is assumed to be due to neural inhibition between two stimuli that interact spatially or within a narrow span of time in the fashion demonstrated in simultaneous brightness-contrast experiments. Three principles are brought together in a comprehensive mathematical model of visual masking. The three principles are: lateral inhibition, the integrated visual response function (VRF), and stimulus decay after its offset over a limited period of time (up to about 100 ms). Block's law is extended to apply to the integration of the VRF, including decay. The three principles combined are sufficient to explain well-known masking phenomena such as metacontrast, and forward and backward masking. The mathematical model presented demonstrates clearly the common underlying basis of all masking. The validity of the lesser documented decay after stimulus offset, a necessary assumption to explain masking effects with stimuli that are delayed relative to each other, is demonstrated with experimental evidence.