Now each of these different approaches can sometimes produce perfectly respectable psychological theories. For example, the popular belief, ‘Once bitten, twice shy’, has its counterpart in the psychological literature on punishment. And some theological views, such as the belief that suffering induces more tolerant attitudes, could easily be the starting-point for a programme of psychological research. So we cannot claim that there is a basic difference between the theories put forward by psychologists and those devised by other students and observers of human nature. The distinguishing feature of experimental psychology is not so much the nature of its theories as the methods used to test their validity. The approach adopted is essentially scientific; a psychological theory has to fit the facts of behaviour as derived from systematic observations taken in carefully controlled conditions. If a theory does not fit the facts it is discarded or revised, no matter how long its history, how appealing its logic, or how convenient its implications. As we shall see, this, emphasis on objectivity and rigorous control narrows the range of behaviour that can feasibly be studied, but in return it produces more solid and reliable conclusions.