Brahma Sutras – According to Shankara 1-3-7
Topic 7 - The person of the size of a thumb is Brahman
शब्दादेव प्रमितः ॥ २४ ॥
śabdādeva pramitaḥ || 24 ||
śabdāt—From the word; eva—itself; pramitaḥ—measured.
24. From the very word (‘Lord’ by which it is referred to in the text) (the being) measured (by the size of the thumb is Brahman).
“The being of the size of a thumb, resides in the centre of the body. (Knowing that) Lord of the past and future, one does not seek to hide oneself any more. This is That” (Kath. 2. 4. 12).
The being referred to is Brahman, because he is spoken of as the Lord or ruler of the past and future. It cannot be the individual soul, though the limitation in size and residence in the centre of the body by themselves might be more applicable in its case.
Moreover in reply to the request of Nachikētas who wanted to know Brahman, Yama refers to this being of the size of a thumb thus: “That which you wanted to know is this.”
हृद्यपेक्षया तु मनुष्याधिकारत्वात् ॥ २५ ॥
hṛdyapekṣayā tu manuṣyādhikāratvāt || 25 ||
hṛdi-apekṣayā—With reference to the heart; tu—but; manuṣya-adhikāratvāt—man (alone) being entitled.
25. But with reference to (the space in) the heart (the Highest Brahman is said to be of the size of a thumb); (and because) man alone is entitled (to the study of the Vedas).
How could the all-pervading Brahman be of the size of a thumb, as stated by the previous Sutra?
Because the space in the heart is of the size of a thumb, therefore Brahman, with reference to Its abiding within that space, is described as being of the size of a thumb.
Since Brahman abides within the heart of all living creatures, why is the ‘thumb’ used as a standard?
Because man alone is entitled to the study of the Vedas and to the different Upāsanās of Brahman prescribed in them, therefore it is with reference to him that the thumb- is used as the standard of measurement.