In the Bhagavad-gītā we are informed that the constitutional nature of the individual entity is spirit soul. He is not matter. As spirit soul, he is part and parcel of the Supreme Soul, the Absolute Truth, the Personality of Godhead. We also learn that it is the duty of the spirit soul to surrender to the Supreme Soul, for only then can he be happy. The last instruction of the Bhagavad-gītā is that the spirit soul should surrender completely unto the Supreme Soul, Kṛṣṇa, and in that way realize happiness.
Here also, Lord Caitanya, answering the questions of Sanātana, repeats the same truth. There is a difference, however. Here Lord Caitanya does not give the information about the spirit soul that is already described in the Bhagavad-gītā. Rather, He begins from the point where Kṛṣṇa ended His instruction. It is accepted by great devotees that Lord Caitanya is Kṛṣṇa Himself, and, as such, He begins His instruction to Sanātana from the point where He ended His instructions to Arjuna in the Gītā.