Buddhi Yoga, the yoga of equanimity, is mentioned in Chapter II of the Bhagavad Gita, Krishna instructs Arjuna that he must renounce attachment to his former friends, teachers, and family and engage himself in battle, which was meant for upholding Dharma.
Sri Krishna reminds Arjuna that the atman (the self) is different from the body and that the “Self neither slays nor is slain” (II:19).
When buddhi (intellect) becomes established in jnana (knowledge), then liberation can be at hand. This knowledge requires equanimity. He says “Treating alike pleasure and pain, gain and loss, victory and defeat, prepare yourself for battle. Thus you will incur no sin.” (II.38)
The evenness of mind attained through buddhi yoga or the disciplined mind is described further by Krishna as the key to liberation: “He who has reached evenness of mind casts off both good and evil deeds in this life. Therefore, strive for yoga, which is skill in action. The wise who have attained evenness of mind renounce the fruits of action. The wise are freed from the fetters of birth and attain the state that is beyond sorrow.” (II:50 -51).
By dwelling in the knowledge that the body is impermanent and by cultivating a state of meditative indifference toward things of the world, the yogi becomes liberated through buddhi yoga.
Evenness of mind is essential in performing action without concern for its fruits. In Samkhya Karika, Ishwara Krishna describes this knowledge as the perception that “I am not this, this is not mine, there is no self which is me.” (64)
Significantly, the three verses in the Bhagavad Gita that refer to the buddhi yoga (II-49, 50 and 51) occur in the chapter described as samkhya yoga.
The samkhya system of philosophy recognizes no creator but gives a theory of evolution from two ultimate uncaused realities, purusha and prakriti. Thus buddhi has a place in the process of evolution.
In the Bhagavad Gita, buddhi yoga, according to commentators is to be taken in the sense of karma yoga in which duties are performed with an equipoise and renouncing all attachments and desires, and not in the sense of jnana yoga. Action with a selfish motive is inferior to buddhi yoga (buddhiyogat, karma durena avaram).