The oldest Upani@sads are written in prose. Next to these we have some in verses very similar to those that are to be found in classical Sanskrit. As is easy to see, the older the Upani@sad the more archaic is it in its language. The earliest Upani@sads have an almost mysterious forcefulness in their expressions at least to Indian ears. They are simple, pithy and penetrate to the heart. We can read and read them over again without getting tired. The lines are always as fresh as ever. As such they have a charm apart from the value of the ideas they intend to convey. The word Upani@sad was used, as we have seen, in the sense of "secret doctrine or instruction"; the Upani@sad teachings were also intended to be conveyed in strictest secrecy to earnest enquirers of high morals and superior self-restraint for the purpose of achievingemancipation. It was thus that the Upani@sad style of expression, when it once came into use, came to possess the greatest charm and attraction for earnest religious people; and as a result of that we find that even when other forms of prose and verse had been adapted for the Sanskrit language, the Upani@sad form of composition had not stopped. Thus though the earliest Upani@sads were compiled by 500 B C., they continued to be written even so late as the spread of Mahommedan influence in India. The earliest and most important are probably those that have been commented upon by S'ankara namely B@rhadara@nyaka, Chandogya, Aitareya, Taittiriya, Is'a, Kena, Katha, Pras'na, Mundaka and Mandukya [Footnote ref 62]. It is important to note in this connection that the separate Upani@sads differ much from one another with regard to their content and methods of exposition. Thus while some of them are busy laying great stress upon the monistic doctrine of the self as the only reality, there are others which lay stress upon the practice of Yoga, asceticism, the cult of S'iva, of Visnu and the philosophy or anatomy of the body, and may thus be respectively called the Yoga, S'aiva, Visnu and S'arira Upani@sads. These in all make up the number to one hundred and eight.