Important philosophical problems derive from the
epistemology of perception—how we can gain knowledge via perception—such as the question of the nature of
qualia. Within the biological study of perception naive realism is unusable. However, outside biology modified forms of naive realism are defended.
Thomas Reid, the eighteenth-century founder of the
Scottish School of Common Sense, formulated the idea that sensation was composed of a set of data transfers but also declared that there is still a direct connection between perception and the world. This idea, called direct realism, has again become popular in recent years with the rise of
postmodernism. The succession of data transfers involved in perception suggests that
sense data are somehow available to a perceiving subject that is the substrate of the percept. Indirect realism, the view held by
John Locke and
Nicolas Malebranche, proposes that we can only be aware of
mental representations of objects. However, this may imply an infinite regress (a perceiver within a perceiver within a perceiver...), though a finite regress is perfectly possible. It also assumes that perception is entirely due to data transfer and information processing, an argument that can be avoided by proposing that the percept does not depend wholly upon the transfer and rearrangement of data. This still involves basic ontological issues of the sort raised by
Leibniz, Locke,
Hume,
Whitehead and others, which remain outstanding particularly in relation to the
binding problem, the question of how different perceptions (e.g. color and contour in vision) are "bound" to the same object when they are processed by separate areas of the brain. Indirect realism (representational views) provides an account of issues such as perceptual contents,
qualia, dreams, imaginings,
hallucinations, illusions, the resolution of
binocular rivalry, the resolution of
multistable perception, the modelling of motion that allows us to watch TV, the sensations that result from direct brain stimulation, the update of the mental image by saccades of the eyes and the referral of events backwards in time. Direct realists must either argue that these experiences do not occur or else refuse to define them as perceptions. Idealism holds that reality is limited to mental qualities while skepticism challenges our ability to know anything outside our minds. One of the most influential proponents of idealism was
George Berkeley who maintained that everything was mind or dependent upon mind. Berkeley's idealism has two main strands,
phenomenalism in which physical events are viewed as a special kind of mental event and
subjective idealism.
David Hume is probably the most influential proponent of skepticism. A fourth theory of perception in opposition to naive realism,
enactivism, attempts to find a middle path between direct realist and indirect realist theories, positing that
cognition is a process of dynamic interplay between an organism's sensory-motor capabilities and the environment it brings forth. Instead of seeing perception as a passive process determined entirely by the features of an independently existing world, enactivism suggests that organism and environment are structurally coupled and co-determining. The theory was first formalized by Francisco Varela, Evan Thompson, and Eleanor Rosch in "The Embodied Mind". ==Spatial representation==