What is Yoga?

17. Yoga systematically teaches man to search for the divinity within himself with thoroughness and efficiency. He unravels himself from the external body to the self within. He proceeds from the body to the nerves, and from the nerves to the senses. From the senses he enters the mind, which controls the emotions. From the mind he penetrates into the intellect, which guides reason. From the intellect, his path leads to the will and thence to consciousness (chitta). The last stage is from consciousness to his self, his very being (atma).

18. Thus, Yoga leads the sadhaka from ignorance to knowledge, from darkness to light, and from death to immortality.

--B.K.S. Iyengar "Light on Pranayama"