[[File:Tesoro Covarrubias.jpg|thumb|Cover of the Tesoros editio princeps (1611).]] The Tesoro de la lengua castellana o española''' (Thesaurus of Castilian or Spanish Language) is a dictionary of the Spanish language, written by Sebastián de Covarrubias in 1611. It was the first monolingual dictionary of the Castilian (Spanish) language, with its lexicon defined in Spanish. The etymological dictionary was among the first of its type published in Europe in a vernacular language. Original publication Sebastián de Covarrubias began writing what would become the Tesoro de la lengua castellana o española in the spring of 1605. He completed the work over the course of three years, writing in Valencia and Cuenca. Covarrubias, who was sixty-six years old when he began the project, decided to reduce the number of words after the letter , fearing he might die before finishing the project. Covarrubias's stated intention was to develop an etymological dictionary to trace the origins of Castilian, modeled on the Etymologiae by Isidore of Seville, who had done the same for Latin. Covarrubias adopted Isidore's idea that the original form of a word is related to its original meaning, so that investigating etymology reveals the origin and deeper meaning of things. The quality of Covarrubias's etymologies were prone to fanciful speculation, in line with other etymological work of the time. He was especially interested in connecting Spanish words to Hebrew, which was considered the original language of humanity before the Tower of Babel. Covarrubias was also aware of contemporary work in lexicography from other countries, including Jean Pallet's Dictionnaire très ample de la langue espagnole et françoise [Very Copious Dictionary of the Spanish and French Language] (Paris, 1604) and Jean Nicot's Trésor de la langue français [Treasury of the French Language] (Paris, 1606). He aimed to give foreigners a sense of the propriety and elegance of the Spanish language, to the honor of the Spanish nation ("dar noticia a los extranjeros del lenguaje español, y de su propiedad y elegancia, que es muy gran honor de la nación española"). Organization The lexicon consists of about 11,000 entries. Including words that do not have their own entry but are defined in sections treating other words, the number of etymologies rises to about 17,000 according to Martí de Riquer. The lexicon features gaps and inconsistencies in alphabetic order. Spelling reflects the instability of written Spanish prior to the establishment of the Spanish Academy, so that a