In his book Objective Knowledge, Karl Popper (1972: 106) outlines a “three world” theory, which serves as the basis for his epistemological arguments:

What I have described as biomedicine's “folk epistemology” is consistent with such an ordering of reality. Disease is located in the body as a physical object or physiological state, and whatever the subjective state of individual minds of physicians and patients, medical knowledge consists of an objective representation of the diseased body. I have argued for an anthropological alternative to such an analysis of medical knowledge, based on a critical examination of how medical practices and ontologies shape the objects of medical attention. However, the difficulties with the objectivist account are more immediately evident when we look closely at illness and its experience. For the person who is sick, as for the clinician, the disease is experienced as present in the body. But for the sufferer, the body is not simply a physical object or physiological state but an essential part of the self. The body is subject, the very grounds of subjectivity or experience in the world, and the body as “physical object” cannot be neatly distinguished from “states of consciousness.”