Peter Mark Roget was nearing his 70th birthday when he was forced to retire from the Royal Society, London's esteemed collection of scientists, so that the younger generation could begin its work [source: Winchester]. At that point, Roget, who was born in 1779, had done many things worthy of a secure legacy. He had developed the log-log slide rule, which allowed mathematicians to work with logarithms long before the calculator made an appearance, and he had published many medical and scientific papers, including several entries for the fledgling Encyclopedia Britannica. He was even indirectly involved with the invention of motion pictures, thanks to his paper on the human eye's ability to create a persistent image even when the image is briefly interrupted.
Instead of resting on his laurels, though, Roget turned to a project that had interested him since the time he was a young man: a scientific ordering of language. Long compelled to make lists of similar words, he envisioned a book that would not define words, but group them according to a classification, such as "space" or "moral powers" [source: Winchester]. The first edition of Roget's Thesaurus was published when Roget was 73, and he oversaw every update until he died at age 90 [source: Upchurch]. Though writer Simon Winchester argues that Roget's book was never intended for the masses, but rather for scholars interested in these classifications, Roget managed to create one of the most enduring reference materials in his retirement years.