Tamāla Kṛṣṇa: "The principles of the Bhagavad-gītā were spoken to Arjuna, and for that matter, to other highly elevated persons, because they were highly advanced compared to ordinary men in other parts of the world. Two plus two equals four. This is a mathematical principle, and it is true both in the infant's arithmetic class and in the master's degree 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 under various circumstances. The higher principles of religion begin with the acceptance of the four orders and the four ranks of social life, as will be explained later. The whole purpose of the mission of incarnations is to arouse Kṛṣṇa consciousness everywhere."
Eight: "In order to deliver the pious and to annihilate the miscreants, as well as to reestablish the principles of religion, I advent Myself millennium after millennium."
Nine: "One who knows the transcendental nature of My appearance and activities does not upon leaving the body take his birth again in this material world, but attains My eternal abode, O Arjuna."
Prabhupāda: This is very nice. If one can understand the principles of appearance and disappearance of God, His activities, so simply by understanding these principles he will be liberated. It is said here that after quitting this body, he is no more coming to take birth again in this material world. So just like a layman does not know how the sun appears and disappears, but an astronomer, he knows very well the movements of the sun, moon, and other planets' appearance and disappearance. This is a science, astronomy. Similarly, there is a science of God by which you can understand how God appears, disappears, how He acts, how He works. Everything is there, but if you are not interested that does not mean that the science of God is false or there is no such science.