WORDS FROM BKS IYENGAR:

About Hatha Yoga:
“ Ha means sun, which is the sun of your body, that is to say your soul, and tha means moon, which is your consciousness. The energy of the sun never fades, whereas the moon fades every month and again from fading comes to fullness. So the sun in all of us, which is our soul, never fades, whereas the mind or consciousness, which draws its energy from the soul, has fluctuations, modulations, moods, ups and downs like the phases of the moon. It is like quicksilver, and as quicksilver cannot be caught by the hand, so we canot easily catch hold of the mind. Yet when consciousness becomes still, and when the energy of consciousness is still, consciousness too is still, and the soul pervades the entire body.
From The tree of Yoga, 1988.

About Yoga Asanas:
“In any yoga pose, two things are required: sense of direction and centre of gravity. Many of us do not think of the sense of direction, yet in each pose, both sense of direction and the centre of gravity need to be maintained. To maintain the centre of gravity, the muscles all have to be aligned with each other.”   
From The tree of Yoga, 1988.