A tradition of natural science books continued in the middle ages from ancient Greek and Roman writings through the early middle ages, when writers such as Isidore of Seville (6th century) and Bede (early 8th century) write treatises on cosmology and calculation of time. Later expanded at Fleury and taken further still by Abbo of Fleury, who lived at Ramsey in the 10th century and was an important intellectual link to the continent, the tradition flourished in Britain's monasteries. To these natural science books belong a tradition of diagrams which incorporate Christian cosmological doctrine into classical theories on the structure of the heavens and earth, uniting geography, physics and computation of time to demonstrate the harmony of creation. Books such as this one (which is in two separate manuscript fragments in the British Library) would have been textbooks for training monks and churchmen. One of the most ancient means of indicating directions was by reference to winds: Homer listed four (equivalent to our north, south, east, and west), Pliny differentiated intermediate points, producing eight, while Aristotle identified twelve. This diagram illustrates the twelve Aristotelian winds, each of which is captioned with two variant names. Mediterranean sailors named winds after the places from which they came, such as Greco (from Greece) to designate the north-east, or Africus for south-west. Some were named after the gods, some after astronomical constellations, while others were identified and personified according to the weather they brought with them. In medieval Christian natural science, the numbers four and twelve (the product of four and three) were considered signs of the harmony of creation and revealed the truth of scripture. Four, the number of winds, humours, essences, and the gospels, indicated the framework of the world's structure. Multiplied by the number of the Trinity, it made twelve, the number of the apostles. Aristotle had, in this way of thinking, access to God's truth and so provided understanding of creation.