Foolish people try to concoct knowledge by manufacturing something in their brains. That is not the real way of knowledge. Knowledge is sabda-pramana, evidence from Vedic literature. Srila Vyasadeva is called Mahamuni. He is also known as Vedavyasa because he has compiled so many sastras. He has divided the Vedas into four divisions-Sama, Rg, Yajur and Atharva. He has expanded the Vedas into eighteen Puranas and has summarized Vedic knowledge in the Vedanta-sutra. He also compiled the Mahabharata, which is accepted as the fifth Veda. Bhagavad-gita is contained within the Mahabharata. Therefore Bhagavad-gita is also Vedic literature (smrti). Some of the Vedic literatures are called srutis, and some are called smrtis. Srila Rupa Gosvami recommends in the Bhakti-rasamrta-sindhu (1.2.101):

Unless one refers to sastra (sruti, smrti and puranadi), one's spiritual activity simply disturbs society. There is no king or government to check people, and therefore society has fallen into a chaotic condition as far as spiritual understanding is concerned. Taking advantage of this chaotic condition, many rascals have appeared and proclaimed themselves incarnations of God. As a result, the entire population is indulging in sinful activities such as illicit sex, intoxication, gambling and meat-eating. Out of many sinful people, many incarnations of God are emerging. This is a very regrettable situation, especially in India.