From the middle of the thirteenth century, Arab philosophers played an essential part in the development of Western Christian thought, philosophical and theological. The number known was relatively small, but two at least were inevitably familiar: Avicenna and Averroes; these had to be reckoned with, and indeed it soon became imperative to choose between them when framing one's philosophy. Now, the importance of Arabic writing to Western thinkers was no sudden occurrence but the result of a long process that had begun nearly a century before the Latin translations of Averroes' principal commentaries were completed (in about 1230). The twelfth century, indeed, witnessed the appearance, in the field of speculation, of a number of philosophic texts translated from the Arabic, following on certain scientific writings of the same provenance. Furthermore, twelfth-century thinkers studied this rich, newly discovered literature with as much zeal as those of the thirteenth century were to do, although inevitably they made a somewhat different use of it. Thus Adelard of Bath, in his Quaestiones naturales, praises the learning and rational method of his Arab teachers; shortly before 1150 Peter the Venerable writes that the ‘Saracens’, as he calls them, are ‘clever and learned men’ whose libraries are full of books dealing with the liberal arts and the study of nature, and that Christians have gone in quest of these.