Pain is difficult to assess because, like many other symptoms, you cannot see it, only feel the results of it. Some researchers have investigated pain by examining the electrical activity of neurones and until fairly recently in the history of pain research, the common belief of medical scientists and lay-people alike, was that the amount of pain a person reported was directly related to the degree of their tissue damage or disease. However, research has shown that a substantial number of people in pain do not have either damage or disease.