In the Upanishads, we get an intelligible body of verified and verifiable spiritual insights mixed with a mass of myths and legends and cosmological speculations relating to the nature and origin of the universe. While the former has universal validity, and has a claim on human intelligence in all ages, the latter forswears all such claim.

All positivistic knowledge contained in any literature, including religious literature, is limited and conditioned by the level of contemporary scientific knowledge. Modification, and even scrapping, of much of this knowledge due to subsequent advances has affected the truth-validity of much of man’s literary heritage, including his religious and philosophical ones.

The spiritual insights of the Upanishads, however, are an exception to the tyranny of time. Subsequent scientific advances have not only not affected their truth-value but have, on the contrary, only helped to reveal the rational basis of their insights and enhance their spiritual appeal. This is no wonder, because these insights are the products of an equally scientific investigation into a different field of experience, namely, the world of man’s inner life.

Reference: The Message of the Upanishads by Swami Ranganathananda (p.4,5)