BG Chap 13 - The description of sādhu and śāstra according to the guru, Kṛṣṇa
The various sages in various Vedic writings have described the knowledge in great detail, whereas Kṛṣṇa says He will describe them in brief. It is also presented by the logical, very decisive words of the Vedānta-sūtras with all reasoning. Although Kṛṣṇa has just said “That is My opinion,” and although He has already established His position as the Supreme Personality of Godhead, He nonetheless refers to two other authorities to substantiate His words. Kṛṣṇa, in the role of guru, thus presents corresponding references to sādhu and śāstra. The Supreme Personality of Godhead, Kṛṣṇa, is the highest authority in explaining this knowledge. Still, as a matter of course, learned scholars and standard authorities always give evidence from previous authorities. Kṛṣṇa is explaining this most controversial point regarding the duality and non duality of the soul and the Supersoul by referring to a scripture, the Vedānta, which is accepted as authority.