The Puruṣa of the size of a thumb is the inner Ātman, always established in the heart of human beings. To Him, entered into the body itself, you do not see although you dwell with Him.

This is one of the most cited descriptions of the Ātman: aṅguṣṭha-mātraḥ (of thumb’s size), located in the hṛdaya (heart). This “inner puruṣa” (antar-ātmā) is always present, but remains invisible due to avidyā (ignorance). The thumb metaphor probably refers to the space in the center of the chest, where the presence of the deepest self is felt. In yoga, this cardiac localization is fundamental for practices like dhāraṇā and dhyāna.