Next to Logic the influence of the preparatory or mathematical sciences falls to be noticed here. Like the prose of ordinary intercourse and the rhymes of the Koran, the verses of the poets were not only collected but also arranged according to special principles of classification,—for example, according to metre. After Grammar Prosody arose. Khalil († 791), the teacher of Sibawaih, to whom the first application of Qiyas to grammatical science was attributed, is said even to have created metrical science. While language came to be regarded as the national, conventional element in poetry, the notion was entertained that what was natural, and common to all populations, would be found in their metre. Thabit ibn Qorra (836–901) therefore maintained, in his classification of the sciences, that metre was something essential, and the study of metre a natural science, and therefore a branch of philosophy. 5. Grammatical science, nevertheless, limited as it was to the Arabic language, retained its peculiarities, upon which this is not the place to enter. At all events, it is an imposing production of the keenly-observing and diligently-collecting Arab intelligence,—a production of which the Arabs might well be proud. An apologist of the 10th century, who was engaged in combating Greek philosophy, said: “He who is acquainted with the subtleties and profundities of Arab poetry and versification, knows well that they surpass all such things as numbers, lines and points, which are wont to be advanced in proof of their opinions, by people who idly dream that they are capable of understanding the essence of things. I cannot see the substantial advantage of things like numbers, lines and points, if, in spite of the trifling profit which may attend them, they do harm to the Faith and are followed by consequences, against which we have to invoke the help of God.” Men would not have their delight in the minutiae of their language disturbed by general philosophic speculations. Many a word-form, originating with the translators of foreign works, was held in detestation by purist Grammarians. The beautiful art of calligraphy, more decorative in its nature than constructive, like Arabic art in general, became developed in noble, delicate forms, and met with a wider expansion than scientific research into the language. In the very characters of the Arabic speech, we may still see the subtlety of the intelligence which formed them, although at the same time we may see a lack of energy, which is observable in the entire development of Arab culture. 2. Ethical Teaching.