Semmelweis established in the 1840s that mortality in a maternity ward fell dramatically when doctors washed their hands between the dissecting room and the delivery bed. The finding is usually told as a story about an establishment refusing evidence, which is partly fair and leaves out the awkward part: Semmelweis could not explain why it worked, germ theory being two decades away, and his colleagues were being asked to accept a causal claim with no mechanism and to concede that they had been killing patients. Neither obstacle excuses the delay. Both help explain it, and they recur whenever an intervention demonstrably works before anyone can say why. Recognising that pattern is more useful than the moral the story is usually told to deliver.