The tragic history of the purging-and-bleeding treatment of yellow fever is an example of the errors of medical thinking in the past. Frequent reexamination of such episodes is necessary if similar errors are to be avoided in the future. When a new remedy is proposed, the favorable reports appear before the unfavorable. Elaborately designed investigations, with precautions to eliminate bias, are necessary in many situations. But dramatic results like those given by insulin in diabetic coma can be observed by the individual physician. If he is trained to be objective and critical his place in the scheme of things will not be taken by the highly organized research team. He makes the decision as to whether a given patient was helped or harmed by a given drug, and it is he who makes the ultimate practical test.