What is Yoga, as per The Katha Upanisad
The Katha Upanisad provides one of the earliest articulations of yoga, and expresses a central way of conceiving yoga, restraint of the senses, likened to the yoking of the vicious horses of a chariot driver -Kath. Up. 3:5.
In this metaphor, the self, atman, rides in a chariot representing the body. The driver is the intelligence or faculty of discriminating wisdom (buddhi), and mind (manas) is the reins. In this Upanisad the young man Naciketas receives instruction from Yama, Lord of Death, on the means of attaining Brahman and immortality. Yama teaches that the wise one may transcend joy and sorrow by the study of what pertains to the self, “set in the secret place of the heart, dwelling in the depth primeval, by considering him as God” Kath. Up. 2:12
Yama instructs that this self is not slain when the body is slain. This is also repeated in Bhagavadgıta 2:19.20a.
Yama describes yoga thus:
When cease the five senses together with the field of consciousness (manas), And the intelligence (buddhi) stirs not
That is the highest course. This is yoga. The firm holding back of the senses. Then one becomes undistracted. Yoga, truly, is the origin and the end.
Kath. Up. 6:10.11