The philosophy of perception is concerned with the nature of perceptual experience and the status of perceptual data, in particular how they relate to beliefs about, or knowledge of, the world. Any explicit account of perception requires a commitment to one of a variety of ontological or metaphysical views. Philosophers distinguish internalist accounts, which assume that perceptions of objects, and knowledge or beliefs about them, are aspects of an individual's mind, and externalist accounts, which state that they constitute real aspects of the world external to the individual. The position of naïve realism—the 'everyday' impression of physical objects constituting what is perceived—is to some extent contradicted by the occurrence of perceptual illusions and hallucinations and the relativity of perceptual experience as well as certain insights in science. Realist conceptions include phenomenalism and direct and indirect realism. Anti-realist conceptions include idealism and skepticism. Recent philosophical works have expanded on the philosophical features of perception by going beyond the single paradigm of vision (for instance, by investigating the uniqueness of olfaction).
Scientific accounts of perception An object at some distance from an observer will reflect light in all directions, some of which will fall upon the corneae of the eyes, where it will be focussed upon each retina, forming an image. The disparity between the electrical output of these two slightly different images is resolved either at the level of the lateral geniculate nucleus or in a part of the visual cortex called 'V1'. The resolved data is further processed in the visual cortex where some areas have specialised functions, for instance area V5 is involved in the modelling of motion and V4 in adding colour. The resulting single image that subjects report as their experience is called a 'percept'. Studies involving rapidly changing scenes show the percept derives from numerous processes that involve time delays. Recent fMRI studies show that dreams, imaginings and perceptions of things such as faces are accompanied by activity in many of the same areas of brain as are involved with physical sight. Imagery that originates from the senses and internally generated imagery may have a shared ontology at higher levels of cortical processing. Sound is analyzed in term of pressure waves sensed by the cochlea in the ear. Data from the eyes and ears is combined to form a 'bound' percept. The problem of how this is produced, known as the binding problem. Perception is analyzed as a cognitive process in which information processing is used to transfer information into the mind where it is related to other information. Some psychologists propose that this processing gives rise to particular mental states (cognitivism) whilst others envisage a direct path back into the external world in the form of action (radical behaviourism). Behaviourists such as John B. Watson and B.F. Skinner have proposed that perception acts largely as a process between a stimulus and a response but have noted that Gilbert Ryle's "ghost in the machine of the brain" still seems to exist. "The objection to inner states is not that they do not exist, but that they are not relevant in a functional analysis". This view, in which experience is thought to be an incidental by-product of information processing, is known as epiphenomenalism. Contrary to the behaviouralist approach to understanding the elements of cognitive processes, gestalt psychology sought to understand their organization as a whole, studying perception as a process of figure and ground.
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