Soon after the publication of Ariel, Rodó started work on what he privately called Proteo, a project which shared a common pedagogic purpose with that 1900 essay. Rodó conceived of the task as a series of volumes but only the first, Motivos de Proteo, was published in his lifetime, in 1909. This is generally seen by critics as his most ambitious book and Rodó himself referred to it in those terms to some of his closer correspondents. In January 1904, he informed Juan Francisco Piquet that he devoted any time he had available to “sculpting” his Proteo, and that he was confident that this would be “mi obra de más aliento, hasta hoy” (my work of greatest scope so far) (OC 1342); in a later letter to Piquet, probably of July 1904, he stated in confidence, and with uncharacteristic immodesty, that the work would “remain” in the literature of the continent, “superando acaso el éxito de Ariel” (perhaps even surpassing the success of Ariel) (1347). In a further correspondence sometime between these two, Rodó provided the same friend with a taster of the contents and style of his book, which would alternate moral philosophy and descriptive prose, tales and apothegms, biographical sketches and psychological observations; the writing is both sophisticated and varied, and draws both on the gravity of traditional Spanish prose and on the lightness and elegance of its French counterpart, generally “recorriendo las inflexiones más diversas del sentimiento y el lenguaje” (covering the most diverse variations of feeling and language) (1343).

Rodó went on to add that the work's essential variety in form and content is nevertheless controlled by a clear structure and an overarching goal. He provided more detail of the overall subject in another contemporary letter (March 1904), this time to Unamuno, saying it related to “‘la conquista de uno mismo’: la formación y el perfeccionamiento de la propia personalidad” (“the conquest of the self”: the formation and refinement of one's own personality) (1393).