In the closing decades of the 20th century, a method of calculating numerical probabilities based on populations-at-risk emerged in public health/epidemiology and then moved into clinical medicine. This new method had its own autonomous social life as it reorganised the fields of clinical perception and clinical practice. This paper documents that revolution in the epistemological basis of medicine by investigating, through primary sources, when and how the social life of a new method undermined the professional status of medicine and changed the doctor-patient relationship.