The difference between what is really present phys- ically (physical or objective reality) and what we actually experience mentally (subjective or phe- nomenal reality) has been an issue at least since the early Greeks. Some accept naive realism, saying that what we experience mentally is the same as what is present physically. Many others, however, say that at least something is lost or gained in the transla- tion from physical to phenomenal experience. A discrepancy between the two types of experience can exist if the sense receptors can respond only partially to what is physically present—for example, to only certain sounds or colors. A discrepancy can also exist if information is lost or distorted as it is being transmitted from the sense receptors to the brain. Also, the brain itself can transform sensory information, thus creating a discrepancy between physical and phenomenal reality. The important question here is, given the fact that there is a phys- ical world and a psychological world, how are the two related? Another question is, given the fact that all we ever experience directly is our own sensa- tions and perceptions, how can we come to know anything about the physical world that presump- tively gave rise to them? We are confronted here with the problem of reification, or the tendency to believe that because something has a name it also has an independent existence.
J. S. Mill (1843/1874) described this fallacy: epistemology because one of its major concerns has been determining how humans gain informa- tion about themselves and their world. The radi- cal empiricist insists that all knowledge is derived from sensory experience, which is somehow regis- tered and stored in the brain. The rationalist agrees that sensory information is often, if not always, the first step in attaining knowledge but argues that the mind must then actively transform this infor- mation in some way before knowledge is attained. Some nativists would say that some knowledge is innate. Plato and Descartes, for example, believed that many ideas were a natural part of the mind.