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. Aristotle believed that a perfect, eternal force existed outside the physical universe and that the philosopher could learn of this force by observing details of the physical world. Part of his idea was great for Christianity: God was the perfect, eternal force. The other part caused problems because case-by-case (empirical) study of the physical world may sometimes find that nature contradicts Church doctrines set down as absolute truths. There was quite a bit of work to do to get Aristotle settled in with Christianity. Some of the pictures in this manuscript hint at the tensions. The picture in the first letter beginning 'On the Heavens and Earth' does not challenge the medieval Christian view of the universe. The earth is in the centre, the celestial spheres (rotating levels of the heavens which hold the planets and stars) surround it and Christ holds it all together. The strange disc (below the letter) made up of profiles of a human face and the animals posing upon the rising scrolls are difficult to explain. It has been suggested that the artist added motifs from other types of science books, such as bestiaries and 'marvels' books (on bizarre inhabitants of other lands).