LOCKE'S PERCEPTUAL THEORY BRIEFLY SUMMARIZED
The major tenets of Locke's theory of perception includes that the brain starts as a "blank slate," a tabula rasa, now know to be a fallacy: the brain and personality do not start as tabula rasa's to be exclusively written upon by environment and learning.
But his belief in tabula rasa led Locke to posit that there is nothing in the mind outside that which is is introduced by the senses. This gives a duality in which the reality of the world is different from our ideas about the world (an idea later upheld by the Sapir-Whorf hypothesis that our birth-language(s) virtually inflexibly constructs our world-view perceptions).
His conclusion is that the world interacts with us through our senses and that the interaction causes the ideas in our minds. Thus ideas are mental (caused by workings of the mind) while reality is extramental (something that exists outside the mental realm: if we fail to think, reality remains).
Objects in reality have primary and secondary properties: primary properties are those that persist whether we sense/perceive them or not while secondary properties exist because of experience and perceptual biases. A classic example of the difference between primary and secondary is salt versus saltiness. Salt has primary chemical composition and texture properties while saltiness has secondary experiential and preference properties. Thus salt is the same for all, NaCl, while saltiness varies between Margarita drinkers and water drinkers. Primary properties are properties of objects in reality while secondary properties are properties of individual perception of primary properties.
How does Locke's theory of perception accord with the general theory of perception based upon today's scientific understanding?
SIZE CONSTANCY: Size constancy is the concept in general perception theory that regardless of proximity, distance, motion or static condition, objects maintain a constancy of perceived size. To illustrate, a man walking away and up a trail will have size constancy--a perception of man-size--regardless of distance across space, speed of motion, static rest or relationship to foreground or background. In other words, when a walking man becomes a small object on the horizon, we do not think he has become Tom Thumb; we have a constant expectation that his size has remained man-size, that he will be 6'4" in the distance if he was 6'4" standing right next to us.
LOCKE: This confirms Locke's precept that properties exist on their own and are extramental (not the product of experience and interpretation of thought). On the other hand, since there is no research suggesting size constancy is taught (to the contrary evidence indicates it is not externally taught), it may refute his precept that we only know objective through sensory experience, since the sensory experience in the example may seem to suggest a physically shrinking man.
RESTORED VISION: There have been instances in modern science when medical procedures have restored vision to children and teens. As a result of these events, it is known that while certain visual aptitudes, like recognize fragments of objects as the whole object, are absent at first, they return after time. The key factor they uncovered in restoring missing visual perceptions of this sort was motion. While not recognizing a visual fragment of an object as part of the whole object, when the fragment was put in motion, recognition rose from 0 percent to 74 percent recognition. [The motion factor supports Gibson's direct, bottom-up perception studies with airplane pilots.]
LOCKE: Restored vision studies refutes Locke's precept that all ideas about reality are extramental: a mental process is occurring in the brain to rectify a perception that is oppositional to the physical nature of the object.
PERCEPTUAL ADAPTATION: Perceptual adaptation is the ability of the brain to adapt to distorted perceptional data by altering it to match the objects primary properties. If a blow to the head caused a distortion of angles, perceptual adaptation would re-alter the distorted angles so they appear in accordance with natural primary properties.
LOCKE: Perceptual adaptation refutes Locke's precept that all ideas about reality are extramental: a mental process is occurring in the brain to rectify a perception that is oppositional to the physical nature of the object, similarly to how restored vision refutes Locke's theoretical precepts.
PERCEPTUAL SET: Perceptual set is the set of stimuli that an individual attends to and recognizes as part of the primary properties of a given stimuli. An individual may be a "Selector" and choose a certain sensory focus of perception, for instance, when certain unpleasant aspects of a scene are ignored and later not recalled as experience. An individual may be an "Interpreter" who can classify and recognize perceptual data, then interpret its meaning, for instance, when an experienced canal builder recognizes a section of canal blocks have been restored because of subtle differences.
LOCKE: Perceptual set supports Locke's precept of secondary properties that are tied with our experience and that provided us with information about the primary properties of the objective world through "simple" and "complex" ideas generated by the secondary properties born of experience.