This Atman, (being) hidden in all beings, is not manifest to all. But it can be realised by all who are trained to inquire into subtle truths, by means of their sharp and subtle buddhi or pure reason’.

We get an echo of this concept of the sheaths covering reality in twentieth-century biology, with respect to the first three, including the Taittiriya Upanisad emphasis of ‘infilling of the succeeding by the preceding’; tenaisa purnah-‘this is infilled by that’. Says the American biologist George Gaylord Simpson (The Meaning of Evolution,)