Upaniṣadic Pyrology

My interest in the Upaniṣadic pyrology (i.e., Feuerlehre or Fire-doctrine) is related to what Erich Frauwallner in his Geschichte der indischen Philosophie states (pp. 174–175):

“Can the mention of the ‘I’ or a soul be completely avoided in the doctrine of deliverance? Was one not straightway forced to speak of a subject who is bound and released? The Buddha knew how to avoid the difficulty with consummate dexterity. Thereby occurred to him one of the basic views on which the Buddhist doctrine of deliverance is built. As soon as, for instance, the law of a psychical organism had formed itself in the circles of philosophical schools, the question arose, as we have shown in the description of Epic philosophy, whether the senses and the psychical organ are reckoned in the sphere of the soul or in the sphere of matter. The answer which was given in different doctrines ran differently. In the dialogue between Manu and Bṛhaspati (Manu-Bṛhaspati-Saṃvādaḥ) we have got acquainted with the doctrine according to which the psychical organism arises out of the soul and returns back to it after deliverance from the cycle of being. The second possibility was, above all, represented by the Sāṃkhya system and its first steps in the Epic, in which all the psychical organs with their functions are ascribed to matter, arise out of it and return back to it. This thought rests on the attempt which had its origin in the Fire-Doctrine of the Upaniṣads and continued in the Epic and especially continued in the Sāṃkhya, to free the soul of all earthly definitions and to remove it from the orbit of origination and causal occurrences. Therefore in the Sāṃkhya, as we shall see in the sequel, all activity is removed to the sphere of matter and the soul is an inactive, pure onlooker. And the same view lies at the basis of the Buddhistic doctrine of deliverance. This is, no doubt, the most important and essential agreement of the oldest Buddhism with the Sāṃkhya.”