Here is great post by Shanna Small.
"Yoga, not the asanas, but the system of inquiry that results in stilling the mind and letting go of what is not true, cannot be done by the spiritually lazy person or the physically lazy person. Anyone who is unwilling to change what is needed to see the truth, whether it is physical or mental, will suffer from the disease of busy-ness. This busy-ness may be in the mind or the body. Stillness unlocks the door to seeing. If water is rushing over rocks, it is hard to see the bottom of the river. When the river is still, we can see. The same is true for us."