In one of the most significant cartouches in his map, Fra Mauro (active ca. 1430-ca. 1459/1464) explains that he proceeded by cross-referencing geographic texts with the accounts of reliable travelers who had personal experience of the places and peoples Fra Mauro describes and portrays visually. A good cosmographer, Fra Mauro maintains, is duty bound to compare the writings of ancient authors such as Pliny (ca. 23-79), Solinus (sec. III), and Ptolemy (ca. 100-ca. 178) with the first-person accounts of contemporary voyagers.

Among such witnesses was Pietro Querini (ca. 1402-ca. 1448), who returned to Venice after surviving a shipwreck in the extreme northern Atlantic; Niccolò de’ Conti (ca. 1395-1469), who visited and described numerous lands in the Far East; the Ethiopian monks who provided Mauro with maps of the African interior; and emissaries of the king of Portugal, who supplied him with the most recent maps of their explorations of the African coast.

Fra Mauro compared their accounts and maps with ancient knowledge and sought a critical method to evaluate and integrate all the diverse sources of information available to him. His world map bespeaks a debate central to Renaissance culture over the clash between ancient authority and modern experience. Fra Mauro argued that cosmographers had a responsibility to weigh, compare, and choose among diverse knowledge claims that sometimes converged and sometimes diverged. When ancient and modern sources disagreed one with another, Fra Mauro held that the direct testimony of his own day must prevail.