Peter Mark Roget did not mean his thesaurus to be read alphabetically. The book he published in 1852, when he was seventy-three, sorted words into about a thousand headings by the idea they express (existence, space, matter, intellect, will, affection), so that someone who knew exactly what they meant but couldn’t find the word could walk in by the meaning and look along the shelf. The alphabetical index went in near the end, at the back, so that people could also use it the other way round. That back half is the part almost everyone now uses: we arrive holding a word we already have and leave with a different one. A book built for people who had the thought and lacked the word turned out to be most loved by people who had the word and were tired of it.