In the 13th-century, western Europe rediscovered Aristotle's books on natural philosophy, or natural science ('Libri naturales'), and this caused an intellectual revolution. Despite being banned in Paris early in the century, they quickly became part of the curricula of universities. Oxford, where this manuscript was probably made, was a leading centre for the study of Aristotle. All students had to read set texts of Aristotle's treatises with commentary, of which this book is an example. It is exceptional, though, because it is quite richly decorated with unusual images. Furthermore, it belonged to Nicholas of Cusa, a German theologian and cardinal, in the 15th century. The makers of this manuscript probably had access to only a few, if any, illustrated copies of Aristotle's scientific texts. Also, it was not an easy book to create pictures for because of its abstract content. The person(s) who planned the programme of images or the artist used resources from other books, mainly Christian texts, or even inspiration from daily life. In some cases, this inventive process worked well to Christianise books which actually presented some conflicts with Christian doctrine. In the picture at the beginning of 'On Sleep and Wakefulness', the upper panel of the woman asleep and a young man approaching her bed may have been taken from a secular source, but the lower image of seven sleeping men probably is an adaptation from stories of saints' lives which included the story of the seven sleepers of Ephesus.