Considering the fact that the Mīmāṃsā, the Vedānta, the Sāṃkhya, and the Yoga schools owe their origin directly to the Vedas, the Brāhmaṇas, and the Upaniṣads, it may be expected that the doctrines of these would have been systematized and put together into the form of the Sūtras earlier than those of the Vaiśeṣika and the Nyāya schools, the essential tenets of which had their beginnings in a later and different kind of literature. This expectation, however, seems to be belied by the fact that the present Sāṃkhya-sūtras have been proved to belong to a very late period, as late as the fourteenth century a.d.; and the Yoga-sūtras are now believed by a number of scholars, following Professors Jacobi and Woods, to be as late as the fourth or fifth century a.d. Now, while the gap of an early systematic work on the Sāṃkhya is filled up by the Sāṃkhya-kārikā, or it may be explained by the surmise that there was an early Sūtra work, either a shorter form of the present one or altogether different from it, which is lost, the Yoga-sūtras are all that we have as a systematic exposition of the Yoga doctrines, and there is no reason to believe that they were preceded by another work of a similar nature. • The question, then, is whether the systematization of such an early school of thought as the Yoga would have been postponed until as late as the fourth or fifth century a.d., and until after the systematization of the doctrines of even the Vaiśeṣika and the Nyāya schools, which began later on, and the Sūtras of which definitely contain a reference to the Yoga doctrines of mystic intuition and concentration.