Mandukya Upanishad, verse 4
Text
स्वप्नस्थानोऽन्तः प्रज्ञाः सप्ताङ्ग एकोनविंशतिमुखः प्रविविक्तभुक्तैजसो द्वितीयः पादः ॥ ४ ॥
svapnasthāno'ntaḥ prajñāḥ saptāṅga ekonaviṃśatimukhaḥ praviviktabhuktaijaso dvitīyaḥ pādaḥ || 4 ||
4. The second quarter (Pāda) is the Taijasa whose sphere (of activity) is the dream, who is conscious of internal objects, who has seven limbs and nineteen mouths and who experiences the subtle objects.
Shankara Bhashya (commentary)
He is called the Svapnasthāna because the dream (state) is his (Taijasa) sphere. Waking consciousness, being associated as it is with many means,1 and appearing2 conscious of objects as if external, though (in reality) they are nothing but states3 of mind, leaves in the mind corresponding4 impressions. That the mind (in dream) without5 any of the external means, but possessed of the impressions left on it by the waking consciousness, like6 a piece of canvas7 with the pictures painted on it, experiences the dream state also as if it were like the waking, is due to its being under the influence of ignorance, desire and their action.8 Thus9 it is said, “(And when he falls asleep) then after having taken away with him (portion of the) impressions from the world during the waking state (destroying and building up again, he experiences dream by his own light)” (Bṛhadāraṇyaka Upaniṣad, 4. 3. 9). Similarly the Atharvana, after introducing the subject with “(all the senses) become one in the highest10 Deva, the mind,” continues “There the god (mind) enjoys in dream greatness”11 (Praśna Upaniṣad). From12 the standpoint of the sense-organs, the mind is internal. He (the Taijasa) is called the Antaḥprajña or conscious of the internal because his consciousness in dream becomes aware of the mental states, which are impressions left by the previous waking state. He is called the Taijasa because he appears as the subject though this (dream) consciousness is without any (gross) object and is of the nature of the essence of light. The Viśva (the subject of the waking state) experiences consciousness associated with gross external objects; whereas, here (in the dream state), the object of experience is consciousness consisting of Vāsanās (the impressions of past experience). Therefore this experience is called the experience13 of the subtle. The rest is common (with the previous Śruti). This Taijasa is the second quarter (of Ātmarn).