Sri Krishna begins His answer.
This answer is at two levels. The first answer is at the level of duty and Dharma, the other is at a deeper level. Duty and Dharma is of course repeated, and Arjuna knows about duty and Dharma and he has rejected already, but it is repeated because in this case duty and Dharma are ultimately coincidental with the deeper knowledge which He is about to expound. Sri Krishna knows that mere appeal to duty or to Dharma will not satisfy Arjuna: he needs a deeper answer because the malady is much deeper. If it was only a question of forgetting duty, one can remind of duty; if it was only a question of forgetting Dharma one can remind of Dharma, but that was not the question here. The question is: duty collides with duty, Dharma collides with Dharma. And he is overpowered by bondage, this real bondage: sorrow. And therefore a deeper question, deeper answer has to be given; and therefore there are two levels of the answer.
In the first place He unmasks Arjuna’s mask of being a very big wise man and a man who now pleads for Sannyasa. He says:
“You are grieving over those who are not fit to be grieved at, yet you speak like a wise man, but the real wise men do not grieve either for the living or for the dead.”(II, 11)
“Never did I not exist, nor did you, nor these kings, nor shall we ever cease to exist in the future.” (II, 12)
“Just as an embodied soul attains childhood, youth and old age through the body, so it attains another body after death. Wise man does not grieve at this.”(II, 13)
“O Son of Kunti! The objects that are perceived by the senses give rise to pleasure and pain, to heat and cold, they are transient. Therefore O Bharata, endure them heroically.” (II, 14)
“O Best among Men! Anyone who is balanced in pleasure and pain, and who is not agitated by the senses and their contact with objects, only such a wise person is fit to attain liberation.” (II, 15)
“There is no existence for the unreal, and the real never ceases to be. Thus the knowers of Reality have ascertained the nature of what is real and what is unreal”. (II, 16)
“That alone by which all this is pervaded is Imperishable because no one can destroy that Immutable Reality.” (II, 17)
“O Bharata! The Self is Imperishable and Immeasurable, but the bodies which are inhabited by the Self are perishable. Therefore prepare to fight.” (II, 18)
“Those who consider the Self as the killer and those who think that it is killed both are ignorant for the Self neither kills nor is killed.” (II, 19)
“The Self is never born, nor does it ever die; having once born before, will it not be born in the future. The Self is unborn, eternal, imperishable and ageless. Though the body is slain, the Self is not killed.”(II, 20)
“O Son of Pritha! If one knows that the Self is indestructible, immutable, unborn, eternal, how can a person kill anyone or cause anyone to be killed.”(II, 21)
“The Self is eternal, all pervading, immutable, stable, and everlasting. Therefore, it cannot be cut, or burnt, or drenched, or dried up.” (II, 24)
“The Self is said to be unmanifest, unimaginable, and unchangeable. Therefore, knowing this to be so, you should not grieve.” (II, 25)
“O Mighty–Armed Arjuna! Even if you think that the Self is born, due to the birth of the body and consider it dead due to the death of the body, you still have no reason to grieve.” (II, 26)
“Death is certain for the one who is born and birth is certain for the one who dies. For this inevitable fact you should not grieve.” (II, 27)
“O Bharata! All created beings are unmanifest in the beginning, manifest in the middle and unmanifest again after the death. So what need is there for lamentation?” (II, 28)
“Some look upon this Self as a wonder, some talk about Him as a wonder, and some hear about Him as a wonder, and yet having heard of Him none is able to understand Him at all.” (II, 29)
“O Bharata! The eternal Self that dwells in the bodies of all beings cannot be slain. Therefore, you should not grieve for any living being.” (II, 30)
This is the first part of the argument.
This is at a higher level. This is because Sri Krishna wants to unmask Arjuna of the mask that he has worn of a wise man, and points out that the real wise man is full of true knowledge, “and if you go to the true knowledge, then look I am giving you the content of that true knowledge. Your argument so far, which you have advanced, does not refer to the Self at all. Your arguments refer to what? They refer to your brethren and your grandfather, and your teachers and your sorrow and your pleasure. The one who is wise speaks of the Self, and in none of your arguments there is any reference to the Self at all! For deciding what you should do, what you should not do, the most important thing is to know what you are. And it is in the Self–knowledge that the true decision can be arrived at”. And that is why Sri Krishna now gives him the first and the basic fact of what can be called ‘true knowledge’.