When Isaac Newton claimed that he only saw further because he stood on the shoulders of giants, he alluded to the long list of geniuses who came before him. The history of science is the story of great discoveries, flashes of intuition that changed the way people saw the world, hard work, and arduous calculation in the laboratory. But who are the most important men and women whose notions and theories have changed the world? From Galileo to Thales, from the ancient world to the present day, it is sometimes those with the simplest ideas who have had the greatest impact. As the study of the physical world becomes more and more specialized, it is sometimes refreshing to uncover a proof that tips accepted wisdom on its head.