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.