Buddhi, the intelligence, is not only a process of thinking but also the rationale behind that thinking. It is the seat of understanding and wisdom.
‘The senses are said to be great, but mind is greater than the senses ; and intelligence is greater than the mind. And that which is greater than even the intelligence, is the self’ (Shloka 3.42).
The buddhi, thus, works as a bridge between the senses and the mind on one hand ; and the soul on the other. When, instead of being guided by the buddhi, the mind becomes a slave of the senses ; one is driven by the desire for the objects of those senses. His actions are determined by such desires. Hence, he is subjected to the bondage of works.
In shlokas from 2.39 to 2.53, Krishna explains as to how, through buddhi yoga, one can free himself from this bondage.
Buddhi is not merely the power to frame and retain concepts ; but it has also the function of recognising, comprehending, reasoning, judging ; and discriminating.
When illumined by consciousness of the self, buddhi is cleared of the ignorance. The wisdom so revealed enables one to discriminate between good and evil, between right and wrong, between truth and falsehood ; and between the self and the non-self.
Whereas the one whose intelligence is disciplined, has a firm determination and a clear aim ; the thoughts of others are scattered and endless. Singlemindedness is a great asset for the achievement of one’s goal.
The work done only for the fruits is far inferior to the work done under the discipline of intelligence. The one who is established in his intelligence casts away, in this very life, both good and evil. Since he has overcome selfishness, he is not attracted towards evil. He rises to an ethical status which transcends both good and evil.
It is important not only as to what one does, but also as to how one does. The spirit behind one’s actions is equally important.
The wise who have united their intelligence with the Divine, free themselves from the bondage of repeated births by renouncing the fruits of their actions. In this manner, they attain that blissful state which is beyond any sorrow. When one’s intelligence crosses the muddy waters of delusion, he becomes indifferent to enjoyments in this life or the next.
Those with superfluous knowledge get unduly influenced by the flowery words of the religious texts. Immersed in material desires, such people aim only at the heaven and good life in the next birth. They always remain bound by rituals. Intelligence of such people is not well-established in the self. They do not focus their mind on the God.
By the practice of buddhi Yoga, one understands that for the one whose intelligence has been illumined by consciousness, ritual observances are not very important. Instead of being led by the letter, he follows the spirit of the scriptures. He renounces all selfish desire and lives life like a sacrifice, offered with true devotion.
He undertakes any work with the understanding that he has right only to action, and never at all to its fruits. Therefore, he performs his prescribed duty without any attachment to the results of his actions. For the one who is so established in buddhi yoga, impending results of the past actions are neutralised and one is freed from the compulsion of future actions and their binding results. Intelligence of such a doer of work gets purified and remains established in the self. He then attains the divine consciousness. Even while remaining ever engaged in the performance of his assigned work, his mind always rests in the Supreme.
Buddhi yoga is the foundation for attaining perfection in all other paths of self-realisation ; because it is not possible to achieve success in any sphere of life, without proper self control and adequate discipline of mind. Krishna has assured that no effort is ever lost on this path of self-improvement. No obstacle on it lasts for long. Even a small move on it saves one from great dangers. In other words, even if one practices yoga only for some time and later abandons it for whatever reason, he still reaps its benefits. Moreover, no negative impact ever accrues from the practice of such yoga.
Krishna has advised that one should be free from all dualities like pleasure and pain, be firmly fixed in purity ; and be free from anxieties of gain and safety. In this manner, he can rise above the three modes of material nature ; and thus be established in the self.
To conclude, when one spontaneously follows the discipline of buddhi yoga, his intelligence gets more and more illumined and touches higher and higher levels of consciousness. As a result, bonds of the egoistic desire are broken. One realises the self, which is his true nature. The false ego dissolves and the sense of separateness vanishes. A vision of harmony is created in which one sees One in all and all in the One.