from the Yoga Sutras of Patanjali
1. Yoga is control of the mind.
Yoga means to control and still the swirling currents of thoughts in the mind.
If you can control the thoughts that arise and still them completely you are able to observe the world clearly and directly without the distortions of the ego. The ability to discipline the chattering mind is what takes us to the state of yoga.
When the turbulence stops and the lake of the mind becomes clear and still our true essence, our self, is reflected. Then the self can abide in its true nature. This is the goal. This is what we want to attain.
When we fail to still our minds, we mistakenly identify ourselves with the activities of the mind and become lost in our thoughts. We loose the true sense of who we are because we have lost sight of our true essence, our real inner self.