Matsya Purana — Nārāyaṇa as Haṃsa in the Cosmic Ocean: Vedic Yajña-Puruṣa and Mārkaṇḍeya’s Vi...
आत्मरूपप्रकाशेन तमसा संवृतः प्रभुः मनः सात्त्विकमाधाय यत्र तत्सत्यमासत //
ātmarūpaprakāśena tamasā saṃvṛtaḥ prabhuḥ manaḥ sāttvikamādhāya yatra tatsatyamāsata //
Though the Lord is self-luminous in His own essential form, He is (as it were) veiled by darkness; and there, having established the mind in sattva (clarity), that Reality is truly realized.
It frames cosmic experience during dissolution/obscuration as dominated by tamas (darkness), while asserting that the Supreme remains self-luminous; realization of truth depends on establishing the mind in sattva.
It implies that right governance and household life should cultivate sattva—clarity, restraint, and discernment—because truth is apprehended through a purified mind rather than through tamasic confusion.
No direct Vastu or iconographic rule is stated; ritually, it supports the principle that purification (sattva) is the prerequisite for effective worship, meditation, and truth-oriented rites.