The Rise of Soma-vaṁśa: Budha’s Birth and Purūravā–Urvaśī; The Origin of Karma-kāṇḍa in Tretā-yuga
स्थालीं न्यस्य वने गत्वा गृहानाध्यायतो निशि । त्रेतायां सम्प्रवृत्तायां मनसि त्रय्यवर्तत ॥ ४३ ॥
sthālīṁ nyasya vane gatvā gṛhān ādhyāyato niśi tretāyāṁ sampravṛttāyāṁ manasi trayy avartata
King Purūravā left Agnisthālī in the forest and returned home, where he meditated on Urvaśī throughout the night. As his meditation continued, the Tretā age began, and thus the principles of the three Vedas—along with the rites of yajña for attaining karmic fruits—arose within his heart.
It is said, tretāyāṁ yajato makhaiḥ: in Tretā-yuga, if one performed yajñas, he would get the results of those yajñas. By performing viṣṇu-yajña specifically, one could even achieve the lotus feet of the Supreme Personality of Godhead. Of course, yajña is intended to please the Supreme Personality of Godhead. While Purūravā was meditating upon Urvaśī, the Tretā-yuga began, and therefore the Vedic yajñas were revealed in his heart. But Purūravā was a materialistic man, especially interested in enjoying the senses. Yajñas for enjoyment of the senses are called karma-kāṇḍīya-yajñas. Therefore, he decided to perform karma-kāṇḍīya-yajñas to fulfill his lusty desires. In other words, karma-kāṇḍīya-yajñas are meant for sensuous persons, whereas yajña should actually be performed to please the Supreme Personality of Godhead. To please the Supreme Personality of Godhead in Kali-yuga, the saṅkīrtana-yajña is recommended. Yajñaiḥ saṅkīrtana-prāyair yajanti hi sumedhasaḥ. Only those who are very intelligent take to saṅkīrtana-yajña to fulfill all their desires, material and spiritual, whereas those who are lusty for sense enjoyment perform karma-kāṇḍīya-yajñas.
This verse indicates that with the commencement of Tretā-yuga, the threefold Vedic knowledge (trayī) again became manifest in the mind, showing that yuga-changes influence how Vedic dharma and remembrance appear in the world.
Śukadeva Gosvāmī speaks this verse to Mahārāja Parīkṣit while narrating Ninth Canto histories and how dharma and Vedic order become established with the progression of the ages.
Even when routine study is disrupted, one can keep returning to sādhana—hearing, chanting, and disciplined contemplation—so that sacred knowledge becomes firmly established in the mind.