Sabdedeva pramitah I.3.24 (87)

From the very word (viz., the term Lord applied to it) the (person) measured (by the size of the thumb) (is Brahman).

An expression from the Kathopanishad is taken up for discussion.

We read in Kathopanishad II-4-12, "The person of the size of a thumb resides in the middle or centre of the body etc." and in II-4-13 "That person, of the size of a thumb is like a light without smoke, lord of the past and of the future, he is the same today and tomorrow. Knowing Him one does not seek to hide oneself any more. This is That."

A doubt arises now whether the person of the size of a thumb mentioned in the text is the individual soul or the Supreme Self (Brahman).

The Purvapakshin or the opponent holds that on account of the statement of the person's size of thumb the individual soul is meant, because to the Supreme Self which is Infinite the Sruti text would not ascribe the measure of a thumb.

To this we reply that the person of the size of a thumb can only be Brahman. Why? On account of the term 'Isana', 'Lord of the past and of the future.' The highest Lord only is the absolute ruler of the past and the future. Further the clause "This is that" connects the passage with that which had been enquired about, and therefore forms the topic of discussion. What had been enquired about by Nachiketas is Brahman. Nachiketas asks Lord Yama, "That which thou seest as neither this nor that, as neither effect nor cause, as neither past nor future, tell me that" (Katha Up. I-2-14). Yama refers to this person of the size of a thumb thus "That which you wanted to know is this."

Brahman is said to be of the size of a thumb, though He is all-pervading, because He is realisable in the limited chamber of the heart of a man.

The epithet 'The Lord of the past and the future', cannot be applied to Jiva at all, whose past and the future is bound by his Karmas and who is not free to possess so much glory.
But how the all-pervading Lord can be said to be limited by the measure of a thumb? The following Sutra gives a suitable answer.

Hridyapekshaya tu manushyadhikaratvat I. 3 25 (88)

But with reference to the heart (the highest Brahman is said to be of the size of a thumb) as man alone is entitled (to the study of the Vedas, to practise meditation and attain Self-realisation).

A qualifying explanation of Sutra 24 is given, and the privilege for Upasana or meditation is discussed.

The measure of a thumb is ascribed to Brahman, although all-pervading, which with reference to his residing within the heart which is generally as big as the thumb. Brahman dwells within the heart of all living beings. The hearts differ according to the animals, some have larger hearts, some have smaller, some are more than a thumb, some are less than a thumb. Why is the 'thumb' used as a standard? Why a man's heart only and not that of any other animal, also? The second half of the Sutra gives an answer - 'on account of man only being entitled'. Man only is entitled to the study of the Vedas and practice of meditation and different Upasanas of Brahman prescribed in them. Therefore the thumb is used as the standard of measurement with reference to him alone.

The aim here is to show the identity of individual soul with Brahman which is inside the body and is of the size of a thumb. The Vedanta passages have twofold purport. Some of them aim in giving a description of the nature of Brahman, some in teaching the unity of the individual soul with the Supreme Soul. Our passage teaches the unity of the individual soul with the Supreme Soul or Brahman, not the size of anything. This point is rendered quite clear further on in the Upanishad. "The person of the size of a thumb, the inner Self, always abides in the heart of men. Let a man draw that Self forth from his body with steadiness, as one draws the pith from a reed. Let him know that Self as 'Bright as the Immortal'." Katha Up. II-6-17.