. Fernando Pessoa described Alberto Caeiro as his teacher, despite the fact that Caeiro was a poetic creation. This article studies the construction of that teacher, from a diachronic approach. Rather than a rounded and defined figure, Caeiro is presented as an author who became more and more real, up until and after his fictitious death, in 1915, and indeed was still being constructed in 1935, the year in which Pessoa himself died. It also aims to explain how it was only in 2008 that Caeiro's genetic dossier became much more complete and to stress the fact that the Caeiro revealed by Pessoa in 1925, in Athena magazine, actually hides aspects of his heteronym.