Arjuna said: Your birth was much later, and Vivasvan’s earlier. How can I then understand that you instructed this first?
Like a young student, Arjuna flings questions at Krishna. The freedom he feels is characteristic of true Guru-sishya relationship.
When did Vivasvan live, wonders Arjuna! Krishna and himself were still young. It was the time of the much later Lunar dynasty. Ikshvaku belonged to the Solar dynasty, of Treta Yuga. Krishna’s was the subsequent Dvapara Yuga, with millennia reigning between them.
Can one born now have any means of having instructed another in the remote past, much less to remember the event vividly? What is this mysterious past-present connection? Anyone would wonder like this.
But did not Krishna right at first say that there was no time when he was not present (2.12)? Soul is eternal. To know this truth is the real object of human life.