The 3 vyāhrtis Bhūh, Bhuvaḥ and Svaḥ, prefix the Gāyatrī Mantra, which Ādi Śaṅkararacharya tells us indicate the nature of the Truth. Oṃ, with which the Mantra begins, is the subtlest of all symbols for, and is the closest indicator of that same Truth. Swamiji gives us further insight into Oṃ, distilled from the vast knowledge of the Upaniṣads.