(All this) has been sung by the Rishis (Seers) in many ways, in distinct metres; and also has been established by the rational words and phrases of Brahma-sūtras (aphoristic statements delineating Brahman, the supreme Reality).
Rishis, given to spiritual reflection and contemplation upon the genesis of experience and knowledge, have sung eloquently about what they experienced and discovered. It is in enchanting poetic compositions, set to different metres, as we find in the Vedas, particularly the Upanishads. These are paramount in every way. Though not inscripted on palm leaves, as was the practice later, the treasure had been preserved indelibly by the Guru- śishya lineages so well.
Moreover, Brahma-sutras, an aphoristic composition dealing with the ultimate Reality, which is the same as the inmost Self, are well known for their unassailable reason and logic. While Upanishads are revelations, Brahma-sutras are well thought out rational presentations. The ancient Upanishads, together with the much later compositions of Brahma-sutras and Bhagavad Gita, constitute Prasthāna-trayam – the three scriptural texts forming the fundamental basis for all our spiritual wisdom.