Akasha

Ākāśa is the first and subtlest of the five gross elements, whose quality is sound. In both Sāṅkhya and Vedānta it is the element that gives room, within which the remaining four abide, and the Taittirīya Upaniṣad declares that from the Self ākāśa arose first of all. At SB 2.5.25 it is said to arise from the transformation of the tāmasa ahaṅkāra, and its quality, sound, is called 'draṣṭṛ-dṛśyayor liṅgam' — the mark of both the seer and the seen — a remarkable definition which treats sound not merely as an object of hearing but as the very ground of the possibility of knowing.

Śrīmad Bhāgavata Mahāpurāṇa 2.5.25 — Sanskrit Wikisource (pinned oldid=200167)