Know the Self (atma) as the master sitting within the chariot which is the body (sarira), know again the understanding (buddhi) as the charioteer and the mind (manas) as the reins.
The senses, they say, are the horses; the objects of sense, what they range over..
He who is ever of unrestrained mind, devoid of true understanding, his sense-desires then become uncontrollable like the wild horses of a charioteer.
But he who is ever of controlled mind, and has true understanding, his sense-desires then are controllable like the good horses of a charioteer . . .
The desires are superior to the senses, the mind is superior to the desires, the intuition (understanding) is superior to the mind, the great Self is superior to the intuition.
Katha Upanishad (I.3.3-6, 10)
This same chariot, the charioteer and the man sitting within the chariot was much later presented to us in the Mahabharata by Sage Vyasa and their conversation is the immortal Bhagavad Gita.