These have been described in my book on Brāhmanism. They were gradually excogitated by independent thinkers, who claimed to be Brāhmans or twice-born men, and nominally accepted the Veda with its Brāhmaṇas, while they covertly attacked it, or at least abstained from denouncing it as absolutely untrue. Such men tacitly submitted to sacerdotal authority, though they really propounded a way of salvation based entirely on self-evolved knowledge, and quite independent of all Vedic sacrifices and sacrificing priests. The most noteworthy and orthodox of the systems propounded by them was the Vedānta[5], which, as I have shown, was simply spiritual Pantheism, and asserted that the one Spirit was the only real Being in the Universe. But the origin of the more unorthodox systems, which denied the authority of both the Veda and the Brāhmans, must also be traced to the influence of the Upanishads. For it is undeniable that a spirit of atheistic infidelity grew up in India almost _pari passu_ with dogmatic Brāhmanism, and has always been prevalent there. In fact it would be easy to show that periodical outbursts of unbelief and agnosticism have taken place in India very much in the same way as in Europe; but the tendency to run into extremes has always been greater on Indian soil and beneath the glow and glamour of Eastern skies. On the one side, a far more unthinking respect than in any other country has been paid to the authority of priests, who have declared their supernatural revelation to be the very breath of God, sacrificial rites to be the sole instruments of salvation, and themselves the sole mediators between earth and heaven; on the other, far greater latitude than in any other country has been conceded to infidels and atheists who have poured contempt on all sacerdotal dogmas, have denied all supernatural revelation, have made no secret of their disbelief in a personal God, and have maintained that even if a Supreme Being and a spiritual world exist they are unknowable by man and beyond the cognizance of his faculties. We learn indeed from certain passages of the Veda (Ṛig-veda II. 12. 5; VIII. 100. 3, 4) that even in the Vedic age some denied the existence of the god Indra. We know, too, that Yāska, the well-known Vedic commentator, who is believed to have lived before the grammarian Pāṇini (probably in the fourth century B.C.), found himself obliged to refute the sceptical arguments of Kautsa and others who pronounced the Veda a tissue of nonsense (Nirukta I. 15, 16). Again, Manu—whose law-book, according to Dr. Bühler, was composed between the second century B.C. and the second A.D., and, in my opinion, possibly earlier—has the following remark directed against sceptics:— ‘The twice-born man who depending on rationalistic treatises (hetu-ṡāstra) contemns the two roots of law (ṡruti and smṛiti), is to be excommunicated (vahish-kāryaḥ) by the righteous as an atheist (nāstika) and despiser of the Veda’ (Manu II. 11).