Chapter VI Sloka06

bNxuraTmaTmnStSy yenaTmEvaTmna ijt>,
AnaTmnStu zÇuTve vteRtaTmEv zÇuvt!

bandhur˜tm˜tmanastasya 
yen˜tmaiv˜tman˜ jita× |
an˜tmanastu þatrutve 
vartet˜tmaiva þatruvat ( 6|06)||

The Self is the friend of the self of him
by whom the self has been conquered by the Self,
but to the unconquered self,
this Self stands in the position of an enemy,
like an (external) foe.

Commentary: Conquer the lower mind through the higher mind. The lower mind is your enemy. The higher mind is your enemy. The higher mind is your friend. If you make friendship with the higher mind you can subdue the lower mind quite easily. The lower mind is filled with Rajas and Tamas (passion and darkness). The higher mind is filled with Sattva or purity.
The Self is the friend of one who is self-controlled, and who has subjugated the lower mind and the senses. But the Self is an enemy of one who has no self-restraint, and who has not subdued the lower mind and the senses. Just as an external enemy does harm to him, so also his own (lower) self (mind) does harm to him. The lower mind injures him severely. The highest Self or Atman is the primary Self. Mind also is self. This is the secondary self.