Katha Upanishad Verse 2.1.12

The seat of the Individual Soul is identified as the heart. Although infinite, it is described as “the size of a thumb” to denote its focused presence within the human form. This Purusha is the master of time—past and future. One who realizes this eternal Lord transcends all fear.

Sanskrit Verse

अङ्गुष्ठमात्रः पुरुषो मध्य आत्मनि तिष्ठति ।
ईशानं भूतभव्यस्य न ततो विजुगुप्सते । एतद्वै तत् ॥ १२ ॥

Transliteration

aṅguṣṭhamātraḥ puruṣo madhya ātmani tiṣṭhati |
īśānaṃ bhūtabhavyasya na tato vijugupsate | etadvai tat || 12 ||

Translation

The Puruṣa (Self), of the size of a thumb, resides in the middle of the body as the lord of the past and the future, (he who knows Him) fears no more. This verily is That.

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Cross-references:

• Dhammapada Verse 3.5 (Heart-Cave / Guhaśayam)