Arjuna said: The sun-god Vivasvän is senior by birth to You. How am I to understand that in the beginning You instructed this science to him?
The Personality of Godhead said: Many, many births both you and I have passed. I can remember all of them, but you cannot, O subduer of the enemy! Although I am unborn and My transcendental body never deteriorates, and although I am the Lord of all living entities, I still appear in every millennium in My original transcendental form.
Whenever and wherever there is a decline in religious practice, O descendant of Bharata, and a predominant rise of irreligion—at that time I descend Myself.
Purport by A.C. Bhaktivedanta Swami Prabhupada:
In the Vedas also it is said that the Lord, although one without a second, manifests Himself in innumerable forms. ll those multiforms are understood by the pure, unalloyed devotees, but not by a simple study of the Vedas (vedesu durlabham adurlabham atma-bhaktau). Devotees like Arjuna are constant companions of the Lord, and whenever the Lord incarnates, the associate devotees also incarnate in order to serve the Lord in different capacities. Arjuna is one of these devotees, and in this verse it is understood that some millions of years ago when Lord Krishna spoke the Bhagavad-gita to the sun-god Vivasvan, Arjuna, in a different capacity, was also present. But the difference between the Lord and Arjuna is that the Lord remembered the incident whereas Arjuna could not remember. That is the difference between the part-and-parcel living entity and the Supreme Lord. The Lord says that He appears in His own body. He does not change His body, as the common living entity changes from one body to another. The conditioned soul may have one kind of body in the present birth, but he has a different body in the next birth. In the material world, the living entity has no fixed body but transmigrates from one body to another. The Lord, however, does not do so. Whenever He appears, He does so in the same original body, by His internal potency. In other words, Krishna appears in this material world in His original eternal form, with two hands, holding a flute. He appears exactly in His eternal body, uncontaminated by this material world.
The purpose of the Vedas is to establish such principles under the order of the Supreme Lord, and the Lord directly orders, at the end of the Gétä, that the highest principle of religion is to surrender unto Him only, and nothing more. The Vedic principles push one towards complete surrender unto Him; and whenever such principles are disturbed by the demoniac, the Lord appears. From the Bhägavatam we understand that Lord Buddha is the incarnation of Kåñëa who appeared when materialism was rampant and materialists were using the pretext of the authority of the Vedas. Although there are certain restrictive rules and regulations regarding animal sacrifice for particular purposes in the Vedas, people of demonic tendency still took to animal sacrifice without reference to the Vedic principles. Lord Buddha appeared to stop this nonsense and to establish the Vedic principles of nonviolence. Therefore each and every avatära, or incarnation of the Lord, has a particular mission, and they are all described in the revealed scriptures. No one should be accepted as an avatära unless he is referred to by scriptures. It is not a fact that the Lord appears only on Indian soil. He can manifest Himself anywhere and everywhere, and whenever He desires to appear. In each and every incarnation, He speaks as much about religion as can be understood by the particular people under their particular circumstances. But the mission is the same—to lead people to God consciousness and obedience to the principles of religion. Sometimes He descends personally, and sometimes He sends His bona fide representative in the form of His son, or servant, or Himself in some disguised form.
The principles of the Bhagavad-gita were spoken to Arjuna, and, for that matter, to other highly elevated persons, because he was highly advanced compared to ordinary persons in other parts of the world. Two plus two equals four is a mathematical principle that is true in the beginner’s arithmetic class and in the advanced class as well. Still, there are higher and lower mathematics. In all incarnations of the Lord, therefore, the same principles are taught, but they appear to be higher and lower in varied circumstances. The higher principles of religion begin with the acceptance of the four orders and the four statuses of social life, as will be explained later. The whole purpose of the mission of incarnations is to arouse "Consciousness of Krishna" (Krishna-bhakti-yoga) everywhere. Such consciousness is manifest and non-manifest only under different circumstances.