That the religious ideas of any epoch tend to flow in the channels dug by the philosophy then prevailing is a commonplace, and it is not surprising, therefore, that in the period between the composition of the Kaṭha Upaniṣad and that of the SK. the various religions which are described in more or less detail in the Upaniṣads and the MBh. are for the most part strongly impregnated with Sāṁkhya doctrines. Not that they accepted the Sāṁkhya scheme wholesale; they accept only so much as is necessary for their purposes and have no hesitation in making modifications or discordant additions of their own. Nevertheless we can discern through the confused welter of systems that the general outlines of the scheme set out by Īśvarakṛṣṇa with its summing up of existence under twenty-five heads were accepted as the standard throughout the period. But how disturbing it would be to all our convictions of historical development if, as has been held, not merely was the outer façade of the Sāṁkhya philosophy maintained intact for all that time, but also there was no change inside. In a lapse of many centuries, during which philosophical speculation was so active and new schools with new ideas and methods were developing, we should expect some change in nomenclature and a great deal of change in the conceptions underlying the apparently unchanging scheme; but there is no general agreement yet about the nature and extent of such changes, if any. Partly, the quality of the evidence is to blame; for we have no exposition of Sāṁkhya teaching which is both certainly authoritative and certainly older than the SK., so that a way is always left open to the retort, when a view other than that contained in the SK.