The name Paul Langevin, inscribed on the walls of many French cities, is a figure of French heritage. He is neither quite legendary, like his illustrious friends Albert Einstein and Marie Curie, nor completely obscure like so many other activist scientists of his generation. He is just famous enough to be unknown. He was depicted as “the most human of scientists” and “a man of deep kindness, constantly putting his devotion at the service of those who suffered”. Langevin combined ethical with intellectual qualities, courage with patience, and reason with generosity and intelligence with humanity. He had all the qualities of a “secular saint”. He was also a “dispersed scholar”, who crossed boundaries and refused to confine himself to a single research area, but who, as a counterbalance, developed a strong aspiration to unification, to the synthesis of knowledge.