Since the beginning of National Insurance in 1911, doctors, employers, employees and government ministries have realised that sick notes are imperfect evidence for running a social security system. They ignored and then rediscovered this fact on a regular basis across the twentieth century. This paper explores why the British welfare state has - despite their imperfections - retained the use of sick notes in one form or another. It uses policy documents, newspapers, internet archives and video sources to show how sick notes have reflected and moulded Britain's relationship with sickness absence from work. It argues that Britain's continued reliance on sick notes shows that gatekeeping procedures required far more than "medical" "proof" of "incapacity".