Water lies beneath the earth. Dig a well in the right spot and it rises to meet you. Krishna says: there is nothing in this world as purifying as knowledge. And this knowledge is not imported from outside — the one who is perfected in yoga discovers it within, in due time.
The reassurance here is twofold. First, knowledge is the supreme purifier — nothing else comes close. Second, it already exists within. The seeker does not need to go to the ends of the earth. Patient, steady practice reveals what was always there, waiting beneath the surface.