While it is known that rod cells in the retina respond to single photons, it has remained an open question whether or not a single photon can actually be perceived by a human being, or whether a few (typically thought to be 5–7) are needed. Jonathan Tinsley of the Research Institute of Molecular Pathology in Vienna, Austria, and colleagues have now shown that we can indeed recognise single photons. The team used a single-photon source based on spontaneous parametric down-conversion and a “two-alternative forced-choice” protocol, in which subjects were asked to distinguish a light stimulus from a blank and were then given feedback about the correctness of their answers. Interestingly, the probability of reporting a single photon is affected by a previous photon having been sent, suggesting a priming process that boosts the visual system on a timescale of seconds.