I have been rereading Professor Paul Thieme's article on the Īśopaniṣad published in the JAOS. In one of his discussions, he seems to claim that the Chāndogya-Upaniṣad is older than the Īśa. Here is that passage:
What is a good modern discussion about the relative chronology of the "older" Upaniṣads? Has anyone discussed Thieme's views? With best regards,