(In the Katha Upanishad) Yama expounds in verse thir­teen, this extraordinary Vedantic discipline for the realisation of the Atman:

यच्छेद्वाङ्मनसी प्राज्ञस्तद्यच्छेज्ज्ञान आत्मनि । ज्ञानमात्मनि महति नियच्छेत्तद्यच्छेच्छान्त आत्मनि ।।१३।।

(Yacchedvanmanasi prajnastadyacchejjnana atmani; Jnanamatmani mahati niyacchettadyacchecchanta atmani.) 1. 3. 13

‘Let the prajna (wise man) merge the speech in the manas, and the manas in the buddhi ; let him merge the buddhi in the great self ( mahat ), and that great self, again, in the Self of peace (the Atman or the Purusha).’

The Atman is significantly characterized as consisting of shanti, peace. Commenting on this, Sankara says:

शान्ते सर्व-विशेष-प्रत्यस्तमित-रूपे अविक्रिये सर्वान्तरे सर्व-बुद्धि-प्रत्यय-साक्षिणि मुख्य आत्मनि

(Shante sarva-visesha-pratyastamita-rupe avikriye sarvantare sarva-buddhi-pratyaya-sakshini mukhya Atmani -)

‘In the peace of the primary (or real) Atman, (which is) char­acterized by the complete cessation of all differentiation (phen­omena), the innermost reality of all, and the witness all the pulsations of buddhi.’

If the innermost Self is all peace, the outermost or the annamaya or the physical self is all noise and distraction. The farther we are from our centre in the Atman, the more become the noise and distraction of our lives. Peace is not in things outside, but within man himself. This peace has to be realised by the devel­opment of the capacity for inner penetration through inner discip­line. The structure of human life becomes steady when it is found­ed on the rock of the eternal Atman within.

Reference: The Message of the Upanishads by Swami Ranganathananda (p.447,448)