Scientific research is an endeavor to enable us — via metaphors, theories, and models — to understand and thereby control reality. We may never be able to fully understand reality, however, because the main tool we use to understand reality, namely our brain, is an inextricable part of reality. At best, so it seems, scientific research may arrive at an understanding of reality as we experience it subjectively, that is, acknowledging the workings of the brain. Understanding the latter is the objective of cognitive neuroscience. Even so, cognitive neuroscientists too use their brain as a tool to understand data. Just as in daily life, this is a potential pitfall because, as an abundance of visual illusions are proof of, what you think you see is not always what you look at.

In this context, human vision research is special in that it takes vision not only as mediating instrument but also as the very topic of study (cf. Rock, 1983). It recognizes that vision is a complex yet fast process that organizes meaningless patches of light on the retina into the objects we perceive, that is, objects with potentially meaningful properties such as shape and spatial arrangement of parts (see Fig. P.1). In other words, when we look at a scene, the objects we perceive constitute the output of vision — not its input.