[In verses three to nine of the second chapter of the Katha Upanishad, chapter three], Yama proceeds to expound to Naciketa the nature of this heroic journey to the summit of character and vision through the field of life and action; in verses three and four, he first speaks of the wonderful equipments for life’s journey that every human being is provided with:

आत्मानं रथिनं विद्धि शरीरं रथमेव तु । बुद्धिं तु सारथिं विद्धि मन: प्रग्रहमेव च ।।३।।

(Atmanam rathinam viddhi shariram rathameva tu; Buddhim tu sarathim viddhi manah pragrahameva ca.)–I. 3. 3

Consider the human body as a chariot, says Yama. The very idea of the chariot, with its wheels, suggests journey; a chariot is not meant to be kept stationary in a shed, but to be put on the road. But the chariot has no motive power in itself; neither has the human body. The chariot gets its motive power from the horses yoked to it. Similarly, the body gets its motive power from the sense-organs consisting of the nervous system and the brain.

The organs of perception and the organs of action convert the animal body into a centre of the most dynamic activity in nature. But at the level of the senses themselves, this activity is mostly uncoordinated and, therefore, not fit for purposes beyond mere organic survival. This co-ordination is found in man in a new faculty of what modern neurologists call ‘imagination’ or insipient mentality (Grey Walter, The Living Brain, p. 2). This is termed manas in Sanskrit; it is defined as सङ्कल्प-विकल्पात्मिका (samkalpa-vikalpatmika), ‘consist­ing of an attitude of may be and may not be’; Swami Vivekananda accordingly translates manas as ‘mind indicisive’. Indian thought treats it in its raw state as on a level with the five sense-organs of perception, and calls it the sixth sense-organ. In the imagery of the chariot, the reins stand for this manas.

The reins involve the charioteer; they have no meaning except in the hands of an intelligent charioteer. In the absence of the charioteer, horses without reins make better sense than horses with reins. For the horses have their own journeys, which are just physical journeys in space and time, the objectives of which are survival and sensate satisfactions. movements subserve the purposes of someone other than themselves. Similarly, the combination of body, sense-organs, and manas points to a reality beyond themselves, a reality which has the capacity to control and direct their movements, like the charioteer in the imagery. This is buddhi or vijnana, reason or enlightened intelligence. ‘Know buddhi as the सारथि (sarathi) or chario­teer’, says the verse.

Even the charioteer, though necessary, is not sufficient; he points to a reality beyond himself, namely, the master of the chariot. The journey is ultimately his; the chariot, the horses, the reins, and the charioteer are only the instruments of his purposes. Simil­arly, the buddhi also points to a reality beyond itself. That reality is the Atman, the Self of man. ‘Know the Atman as the master of the chariot’, says the verse.

This journey towards universality and fulfilment through the development of spiritual intelligence forms the theme of the verses five to eight, of the third chapter of this Upanishad:

यस्तु विज्ञानवान् भवति समनस्क: सदा शुचि: । स तु तत्पदमाप्नोति यस्माद् भूयो न जायते ।।८।।

(Yastu vijnanavan bhavati samanaskaḥ sada suchih; Satu tatpadamapnoti yasmad bhuyo na jayate.)–1. 3. 8

‘He who is possessed of right understanding, with manas held and ever pure, reaches that goal whence there is no birth (return to worldliness) again.’

Life’s journey, to be successful, needs the contribu­tion of all the constituents of the personality: the body, the senses, the manas, the buddhi and the Self; each of these plays a signific­ant part in this journey. But the most important thing is to ensure that the initiative and control pass from the senses to the buddhi through the manas. This cannot happen unless the buddhi and the manas are trained and disciplined into their true forms. The true form of the manas is its pure state when it is aligned with buddhi, and ceases to be a mere appendage of the senses; then alone it initially opposite forces of the senses and the buddhi. The true form of the buddhi is its pure state as Reason, when it is independent of the manas and sense-organs. It then reflects the pure light of awareness of the Atman behind, the true Self, and becomes possessed of discrimination and sound judge­ment.

Reference: The Message of the Upanishads by Swami Ranganathananda (p.409-415)