“Details aside, the frequency-of-seeing experiment brings forward a beautiful idea: the probabilistic nature of our perceptions reflects the physics of random photon arrivals. An absolutely crucial point is that Hecht, Shlaer, and Pirenne chose stimulus conditions such that the five to seven photons needed for seeing were distributed across a broad area on the retina, an area that contains hundreds of photoreceptor cells. Thus, the probability of one receptor (rod) cell receiving more than one photon is very small. The experiments on human behavior therefore indicate that individual photoreceptor cells generate reliable responses to single photons. In fact, vision begins (as we discuss in more detail soon) with the absorption of light by the visual pigment rhodopsin, and so sensitivity to single photons means that each cell is capable of responding to a single molecular event. This is a wonderful example of using macroscopic experiments to draw conclusions about single cells and their microscopic mechanisms.”