This chronological chaos naturally affects the value of literature as a record of the development of thought. We are in danger of moving in a vicious circle: of assigning ideas to an epoch because they occur in a certain book, while at the same time we fix the date of the book in virtue of the ideas which it contains. Still we may feel some security as to the sequence, if not the exact dates, of the great divisions in Indian religious literature such as the period of the Vedic hymns, the period of the Brâhmaṇas, the rise of Buddhism, the composition of the two great epics, and the Puranas. If we follow the opinion of most authorities and accept the picture of Indian life and thought contained in the Pali Tripitaka as in the main historical, it seems to follow that both the ritual system of the Brâhmaṇas and the philosophic speculations of the Upanishads were in existence by 500 B.C.[140] and sufficiently developed to impress the public mind with a sense of their futility. Some interval of mental growth seems to separate the Upanishads from the Brâhmaṇas and a more decided interval separates the Brâhmaṇas from the earlier hymns of the Rig Veda, if not from the compilation of the whole collection[141]. We may hence say that the older Upanishads and Brahmaṇas must have been composed between 800 and 500 B.C. and the hymns of the Rig Veda hardly later than 1000 B.C. Many authorities think the earlier hymns must date from 2000 rather than 1000 B.C. but the resemblance of the Rig Veda to the Zoroastrian Gathas (which are generally regarded as considerably later than 1000 B.C.) is plain, and it will be strange if the two collections prove to be separated by an interval of many centuries. But the stage of social and religious culture indicated in the Vedic hymns may have begun long before they were composed, and rites and deities common to Indians and Iranians existed before the reforms of Zoroaster[142]. It may seem that everything is uncertain in this literature without dates or authors and that the growth of religion in India cannot be scientifically studied. The difficulties are indeed considerable but they are materially reduced by the veneration in which the ancient scriptures were held, and by the retentiveness of memory and devotion to grammar, if not to history, which have characterized the Brahmans for at least twenty-five centuries. The authenticity of certain Vedic texts is guaranteed not only by the quotations found in later works, but by treatises on phonetics, grammar and versification as well as by indices which give the number of words in every book, chapter and verse. We may be sure that we possess not perhaps the exact words of the Vedic poets, but what were believed about 600 B.C. to be their exact words, and there is no reason to doubt that this is a substantially correct version of the hymns as recited several centuries earlier[143].