14. Here, Lord Krishna is describing the cycle of nature. Rain begets grains. Grains are eaten and transformed into blood. From blood, semen is created. Semen is the seed from which the human body is created. Human beings perform yajnas, and these propitiate the celestial gods, who then cause rains, and so the cycle continues.
15. The Vedas emanated from the breath of God: asya mahato bhūtasya
"The four Vedas—Ṛig Veda, Yajur Veda, Sāma Veda, Atharva Veda—all emanated from the breath of the Supreme Divine Personality." In these eternal Vedas, the duties of humans have been laid down by God himself. These duties have been planned in such a way that through their performance materially engrossed persons may gradually learn to control their desires and slowly elevate themselves from the mode of ignorance to the mode of passion, and from the mode of passion to the mode of goodness. These duties are enjoined to be dedicated to him as yajña. Hence, duties consecrated as sacrifice to God verily become godly, of the nature of God, and non-different from him.
The Tantra Sar states yajna to be the Supreme Divine Lord himself:
In the Bhagavatam, Shree Krishna declares to Uddhav: yajno 'haṁ bhagavattamaḥ , the Son of Vasudev, am Yajna." The Vedas state: yajno vai viṣhṇuḥ "Yajna is indeed Lord Vishnu himself." Reiterating this principle, Shree Krishna says in this verse that God is eternally present in the act of sacrifice.
16. Chakra, or cycle, means an ordered series of events. The cycle from grains to rains has been described. All members of this universal wheel of action perform their duties and contribute to its smooth rotation. Since we also partake of the fruits of this natural cycle, we too must do our bounden duty in the chain.
We humans are the only ones in this chain who have been bestowed with the ability to choose our actions by our own free will. We can thus either contribute to the harmony of the cycle or bring about discord in the smooth running of this cosmic mechanism. When the majority of the people of human society accept their responsibility to live as integral parts of the universal system, material prosperity abounds and spiritual growth is engendered. Such periods become golden eras in the social and cultural history of humankind. Conversely, when a major section of humankind begins to violate the universal system and rejects its responsibility as an integral part of the cosmic system, then material nature begins to punish, and peace and prosperity become scarce.
The wheel of nature has been set up by God for disciplining, training, and elevating all living beings of varying levels of consciousness. Shree Krishna explains to Arjun that those who do not perform the yajña enjoined of them become slaves of their senses and lead a sinful existence. Thus, they live in vain. But persons conforming to the divine law become pure at heart and free from material contamination.
17. Only those who have given up desires for external objects can rejoice and be satisfied in the self. The root of bondage is our material desires, "This should happen; that should not happen." Shree Krishna explains a little further ahead in this chapter that desire is the cause of all sins, consequently, it must be renounced. As explained previously (in the purport of verse 2.64), it must be borne in mind that whenever Shree Krishna says we should give up desire, he refers to material desires, and not to the aspirations for spiritual progress or the desire to realize God.
However, why do material desires arise in the first place? When we identify the self with the body, we identify with the yearnings of the body and mind as the desires of the self, and these send us spinning into the realm of Maya. Sage Tulsidas explains:
"When the soul separated itself from God, the material energy covered it in an illusion. By virtue of that illusion, it began thinking of itself as the body, and ever since, in forgetfulness of the self, it has been experiencing immense misery."
Those who are illumined realize that the self is not material in nature, but divine, and hence imperishable. The perishable objects of the world can never fulfill the thirst of the imperishable soul, and therefore it is a folly to hanker after those sense-objects. Thus, self-illumined souls learn to unite their consciousness with God and experience his infinite bliss within them.
The karm (duties) prescribed for the materially conditioned souls are no longer applicable to such illumined souls because they have already attained the goal of all such karm. For example, as long as one is a college student, one is obliged to follow the rules of the university, but for one who has graduated and earned the degree, the rules now become irrelevant. For such liberated souls, it is said: brahmavit śhruti mūrdhnī "Those who have united themselves with God now walk on the head of the Vedas," i.e. they have no need to follow the rules of the Vedas any longer.
The goal of the Vedas is to help unite the soul with God. Once the soul becomes God-realized, the rules of the Vedas, which helped the soul to reach that destination, now no longer apply; the soul has transcended their area of jurisdiction. For example, a pundit unites a man and woman in wedlock by performing the marriage ceremony. Once the ceremony is over, he says, "You are now husband and wife; I am leaving." His task is over. If the wife says, "Punditji, the vows you made us take during the marriage ceremony are not being followed by my husband, the pundit will reply, "That is not my area of expertise. My duty was to get you both united in marriage and that work is over." Similarly the Vedas are there to help unite the self with the Supreme Self. When God-realization takes place, the task of the Vedas is over. Such an enlightened soul is no longer obliged to perform the Vedic duties.