“In dorso colorem habet inter viridem et ceruleum…”

Like other contemporary encyclopaedists of his time, Thomas of Cantimpré (1200 ca.–1270/72) used a vast number of sources in his Liber de natura rerum (completed between 1241 and 1260 ca.), which he meticulously selected to copy, cut and ‘paste’ in order to create a solid, well-argued, coherent and ‘Dominican’ discourse on nature. Among these auctoritates, the friar also uses a mysterious and anonymous libellum, which he qualifies as “liber rerum,” in his work. Consequently, the paper explains this auctoritas through a careful consideration of all the objective aspects that can be acquired from the Liber de natura rerum. Secondly, the work shows how the anonymous source was Thomas’ privileged vehicle through which to introduce in his encyclopaedia ‘alternative’ information borrowed from non-canonical sources (direct observations, personal experiences, etc.). The analysis therefore identifies the particular textual typology of the anonymous libellum, while also demonstrating how the friar of Cantimpré was a curious and actual auctor on nature, observing everyday reality directly and thereby distinguishing himself from his contemporary compilatores.