Purport

This is a very interesting conversation. In earlier Shloka Bhagvan Shri Krishna told how he taught Yoga first to Vivasana, the deity of light. Arjuna got confused and asked a very innocent question. He told, O Krishna, Vivasana is the ancient deity and existed at the start of the creation and you took birth in our time. You might be having similar age as mine, how then you taught Yoga to Vivasna, the ancient deity.

Bhagvan Shri Krishna gave a very clear answer. Bhagvan explained we have spent many births. Only difference is that He(Bhagvan Krishna) knows them all but Arjuna does not. A normal human cannot remember anything from past lives and he ends up thinking that this life is the only life, which is not true.