Parallel (geometry) as a pair of straight lines in a plane which do not meet appears as Definition 23 in Book I of Euclid's Elements. Alternative definitions were discussed by other Greeks, often as part of an attempt to prove the parallel postulate. Proclus attributes a definition of parallel lines as equidistant lines to Posidonius and quotes Geminus in a similar vein. Simplicius also mentions Posidonius' definition as well as its modification by the philosopher Aganis. At the end of the nineteenth century, in England, Euclid's Elements was still the standard textbook in secondary schools. The traditional treatment of geometry was being
Transmission of the Greek Classics referred as translating into Syriac the chief medical authorities as well as parts of Aristotle's "Organon". After studying at Baghdad under Yahya he visited Alexandria and returned, not only with the training given at what was then the first medical school, but with a good knowledge of Greek which he employed in making translations in Syriac and Arabic. Later the Caliph al-Mamun also sent emissaries to the Byzantines to gather Greek manuscripts for his new university, making it a center for Greek translation work in the Arab world. At first only practical works, such as those on medicine and technology