Švetasvatara Upanisad

The Švetasvatara, no doubt does not belong to the series of the more ancient Upanisad, or of those which preceded the foundation of philosophical systems; for it shows in many passages an acquaintance with them, introduces the Vedanta, Sankhya and Yoga, by their very names, mentions the reputed founder of the Sànkhya, Kapila and appears even to refer (in the second verse of the first chapter), to doctrines which have been always considered as heterodox. It must have been always considered as heterodox. It must have been composed at a time when the whole social and political system of the Bràhmanas was completed, when the fiction of the great Kalpas had been adopted, and when the belief of the heroic times in the coequal power of the three great gods, Brahmà, Visnu, and Šiva, had already been abandoned for sectarian doctrines, which are characterized by assigning to one of these gods all the attributes of the other. Here, it is Šiva, or Rudra who not only is declared the creator, preserver, and destroyer of religious belief, but is even identified with the Brahman or supreme spirit of philosophy.
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