Peter Mark Roget was the perfect man to bring some scientific rigor into the art of language. Born in London, the son of a Swiss clergyman, he went on to study medicine and enter the Royal Society of Medicine, and invent the slide rule for calculating square roots and powers of numbers. Throughout, he published prolifically: works on tuberculosis, works the anesthetic effects of nitrous oxide, and numerous other topics in contributions for encyclopedias. The writing helped him escape the unfortunate early deaths in his family, as well as its history of mental illness.

On this day, April 29, at the age 73  Roget published his Thesaurus of English Words and Phrases, Classified and Arranged so as to Facilitate the Expression of Ideas. He had been holding on to the manuscript for 47 years at that point, using it to enhance his own writing.

The 15,000 words in the Thesaurus were not arranged alphabetically, but by concept. Roget’s aim was not just to provide synonyms, but to explain on the ideas by listing out similar, related terms. Roget’s six classes contained 1002 concepts by which the terms were arranged, all according to Aristotelian and Leibnizian principles.