In his Natural History, the Roman author Pliny the Elder (AD 23–79) had included a vast amount of information on medical substances, borrowing from various sources, including botanist Theophrastus, medical authors Heraclides and Sextius Niger, and even from folk medicine based on magic. At the end of the fifteenth century, humanists like Niccolò Leoniceno began to write about the errors of Pliny and other doctors (de Plinii et aliorum medicorum erroribus liber). This task of revision was possible because Leoniceno had then access to Greek manuscripts by Dioscorides and Galen, sources that were mostly unavailable to Western readers in the Middle Ages.