Īśvara, the Supreme Personality of Godhead, is situated in everyone's heart. He is supervising what kind of yantra, or machine, this living entity requires for fulfilling his desires. That is karma. He is engaged in such karma, and he is manufacturing by his karma another body. Everything is going on by the laws of nature. Prakṛteḥ kriyamāṇāni guṇaiḥ karmāṇi sarvaśaḥ (BG 3.27). Guṇaiḥ. By different guṇas—sattva-guṇa, rajo-guṇa, tamo-guṇa; goodness, passion and ignorance—very subtle machineries are going on. They are all material.
So here it is said, therefore, dravya-sphuraṇa-vijñānam. Dravya means physical. Dravya-jñāna. Dravya-jñāna means physical knowledge. And brahma-jñāna means spiritual knowledge. So here it is said, dravya-sphuraṇa. The material, physical, phenomenal atmosphere is developing one after another. The medical science, they are trying to study different cells; but what from the cells came into action? That is by the influence or by the manipulation of prakṛti. Prakṛteḥ kriyamāṇāni (BG 3.27). And prakṛti is working under the direction of Supreme Lord, Kṛṣṇa. Mayādhyakṣeṇa prakṛtiḥ (BG 9.10). Therefore, ultimately, by the superintendence of the Supreme Personality of Godhead everything is going on. But how it is going on, that we cannot explain. We have got limited knowledge. Therefore śāstra says, "Don't try to speculate, because you are imperfect, but things are going on like this. Try to understand." Acintyāḥ khalu ye bhāvā na tāṁs tarkeṇa yo. . . Simply by your so-called argument and logic you cannot understand. You have to hear from the authorities that "Things are going on like this." This is called śruti, Veda-jñāna. Śruti, Veda, perfect knowledge is called śruti. Śrutibhiḥ pratipanna, pratiprapanna(?). What is established by the śruti, by the Vedic knowledge, that we have to accept. Otherwise there is no other way.
So this is even in the material field of activities, and what to speak of the spiritual. Therefore we have to hear from the authorities of śruti. Authorities of śruti. . . Kṛṣṇa is the original speaker of śruti, or Veda knowledge. Therefore Vedic knowledge is called apauruṣeya. It is not manufactured or concocted by some mental speculator. It is coming from the Supreme Personality of Godhead, and it was imparted first of all to Lord Brahmā, not by personal presentation, but through the heart. Brahmā is the original person to understand Vedic knowledge. So after his birth he meditated for hundreds of years, and by meditation, through his heart the knowledge of Vedas, śruti, became revealed. Tene brahma hṛdā ādi-kavaye (SB 1.1.1). Ādi-kavi means Brahmā, and Brahman means Vedic knowledge. So we have to accept in that way. There is no physical science which can ascertain all this, how the things are taking place. But they are coming. In this way we have to learn from the śruti.