Why is the search for the basic unit of knowledge possible and necessary, even when separated from larger philosophical and scientific questions? There are three main reasons. First, one such problem deals with the process by which knowledge is gained about the external world and rational beings interact with their external environment. A second problem examines the existence or ontology of this external world: is it real or an artifact of the mind? Thirdly, another area of inquiry concerns the degree to which we can trust our senses. Are the inputs we receive from the external world “true” representations of the external reality?