Vedantasara - Verse 49
Sanskrit
वनवृक्षतदवच्छिन्नाकाशयोर्जलाशयजलतद्गतप्रतिबिम्बाकाशयो- र्वाधारभूतानुपहिताकाशवदनयोरज्ञानतदुपहितचैतन्ययो- राधारभूतं यदनुपहितं चैतन्यं तत्तुरीयमित्युच्यते “शान्तं शिवमद्वैतं चतुर्थं मन्यन्ते (स आत्मा स विज्ञेयः)” (माण्डू उ ७) इत्यादिश्रुतेः
Swami Nikhilananda's Translation
Like the unlimited Akasa which is the substratum of the Akasa enclosed by the forest and the trees, or of the Akasa which is reflected in the water and the reservoir, there is an unlimited Consciousness which is the substratum of the aggregate and the individual ignorance as well as of the Consciousness (Isvara and Prajna) associated with them. This is called the “Fourth”. As in such Sruti passages as, “That which is (tranquil), auspicious and without a second, That the wise conceive of as the Fourth aspect. (He is the Self; He is to be known)” (Mand. Up. 7).
Index: Vedantasara

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