So this was a discussion between Arjuna and Kṛṣṇa in the Battlefield of Kurukṣetra. So the subject of discussion was that although the battle was declared, Arjuna, when he actually found that "On the other side there are my relatives," how he could slay them? Kṛṣṇa advised that "Everyone must execute his prescribed duty without consideration of any personal loss or gain." According to Vedic civilization, there are four divisions of the society. Everywhere the same divisions are there all over the world. This is very natural. Just like we can study from our own body, there is head, there is arm, there is belly, and there is leg, similarly, in the society there must be a class of men who should be considered as brain, another class of men should be there who will protect the society from danger, another class of men will be expert in producing food grains and give protection to the cows and make trade, so. And the rest class of men, namely who cannot work as brain neither can work as protector from danger, nor they can produce food grains or give protection to the cows, they are called śūdras—as you cannot avoid, to make your body complete, the brain department, the arms department, the belly department and the walking or working department.
So Arjuna belonged to the group of men who was meant for giving protection to the society. So when he was declining to fight, Arjuna, when he was declining to fight, at that time Kṛṣṇa gave him advice that "It is your duty to fight." So generally killing is not good at all, but when there is enemy, aggressor, then to kill the aggressor is not sin. So the other party in the Battlefield of Kurukṣetra, they became aggressor to the Arjuna's party. Now, this is the set up of the Bhagavad-gītā. The real purpose is to instruct Arjuna about spiritual understanding.
So spiritual understanding means first of all to know what is spirit. If you do not know what is spirit, then where is spiritual understanding? People are too much engrossed with the body. That is called materialism. But when you understand what is spirit and you act accordingly, that is called spiritualism. So Arjuna was hesitating to fight with the other party because he had bodily relationship with them. So between Arjuna and Kṛṣṇa there was discussion, but that was friendly discussion. Therefore, when Arjuna understood that simply friendly discussion could not solve the problem, he became His disciple. Kṛṣṇa... Arjuna surrendered to Kṛṣṇa, śiṣyas te 'ham śādhi māṁ prapannam: (BG 2.7) "My dear Kṛṣṇa, so long we are talking as friend. Now I become Your regular disciple. Kindly save me by instruction. What I have to do?" Therefore, when this stage was arrived, Kṛṣṇa is advising Arjuna as follows: śrī-bhagavān uvāca. Now, it is said here... Who is saying to Arjuna? The author or the recorder of Bhagavad-gītā... Bhagavad-gītā was spoken by Kṛṣṇa. It was a discussion between Kṛṣṇa and Arjuna, and it was recorded by Vyāsadeva, and later on it became a book. Just like when we speak it is recorded and later on it is published as a book. Therefore in this book it is said, bhagavān uvāca. Vyāsadeva is the writer. He does not say that, "I speak." He says, bhagavān uvāca, and the "The Supreme Personality of Godhead said."