Verse 1.2.20

अणोरणीयान्महतो महीयानात्मास्य जन्तोर्निहितो गुहायां ।
तमक्रतुः पश्यति वीतशोको धातुः प्रसादान्महिमानमात्मनः ॥ २० ॥

aṇoraṇīyānmahato mahīyānātmāsya jantornihito guhāyāṃ |
tamakratuḥ paśyati vītaśoko dhātuḥ prasādānmahimānamātmanaḥ || 20 ||

20. Subtler than the subtle, greater than the great, in the heart of each living being, the atman reposes. One free from desire, with his mind and the senses composed, sees the glory of the atman and becomes absolved from grief.

Shankara’s Commentary:

Com.—How then does one know the atman is explained? Subtler than the subtle, i.e., subtler than grain, etc.; greater than the great, i.e., greater than things of great dimensions, such as the earth (whatever thing is in the world, that is known to exist only by virtue of the eternal atman; divorced from the atman it becomes a non-entity; therefore, this atman alone is subtler than the subtle and greater than the great, because all names, forms and actions are only conditions imposed upon it). This atman is seated, as the atman, in the heart of every living creature, irons Brahma down to the worm. That atman to whose realisation, hearing, thought and meditation are indicated as aids; one free from desire, i.e., one whose intelligence has been diverted from all external objects, either of this world or of the world to come (when he is so,—the mind and the senses which are called Dhatus.—because they suppprt the body, become composed); sees, i.e., directly realises, in the form ‘I am he’, the glory of the atman, devoid of increase or diminution due to Karma; and, therefore, he becomes absolved from grief.