Bhagavad Gita 6.6 · Bandhur Atmatmanas Tasyaबन्धुरात्माऽऽत्मनस्तस्य येनात्मैवात्मना जितः।अनात्मनस्तु शत्रुत्वे वर्तेतात्मैव शत्रुवत् ॥

Conquer the lower mind through the higher mind. The lower mind is your enemy. The higher mind is your friend. If you make friendship with the higher mind you can subdue the lower mind ite 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 selfcontrolled, and who has subjugated the lower mind and the senses. But the Self is an enemy of one who has no selfrestraint, 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.