And there was light. Vision starts when photons reflected from objects in the world impinge on the retina. Although this may seem rather clear to us right now, it took humanity several centuries, if not more, to arrive at this conclusion. The compartmentalization of the study of optics as a branch of physics and visual perception as a branch of neuroscience is a recent development. Ideas about the nature of perception were interwoven with ideas about optics throughout antiquity and the middle ages. Giants of the caliber of Plato (~428–~348 BC) and Euclid (~300 BC) supported a projection theory according to which cones of light emanating from the eyes either reached the objects themselves or met halfway with other rays of light coming from the objects, giving rise to the sense of vision. The distinction between light and vision can be traced back to Aristotle (384–322 BC) but did not reach widespread acceptance until the investigations of properties of the eye by Johannes Kepler (1571–1630).