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Śrīmad-Bhāgavatam
[Canto 6, Ch. 1]
“Unto those great souls who have implicit faith in both the Lord and the spiritual master, all the imports of Vedic knowledge are automatically revealed.” The  Vedas  enjoin,  tad-vijñānārthaṁ sa gurum evābhigacchet : one must approach a spiritual master who has full knowledge of the  Vedas  and be faithfully directed by him in order to become a devotee of the Lord. Then the knowledge of the  Vedas  will be revealed. When the Vedic knowledge is revealed, one need no longer remain in the darkness of material nature.
According to his association with the material modes of nature—goodness, passion and ignorance—a living entity gets a particular type of body. The example of one who associates with the mode of goodness is a qualified  brāhmaṇa . Such a  brāhmaṇa  knows past, present and future because he consults the Vedic literature and sees through the eyes of  śāstra  ( śāstra-cakṣuḥ ). He can understand what his past life was, why he is in the present body, and how he can obtain liberation from the clutches of  māyā  and not accept another material body. This is all possible when one is situated in the mode of goodness. Generally, however, the living entities are engrossed in the modes of passion and ignorance.
In any case, one receives an inferior or superior body at the discretion of the Supreme Personality of Godhead, Paramātmā. As stated in the previous verse:
manasaiva pure devaḥ pūrva-rūpaṁ vipaśyati anumīmāṁsate 'pūrvaṁ manasā bhagavān ajaḥ
Everything depends on  bhagavān , or  ajaḥ , the unborn. Why doesn't one please Bhagavān to receive a better body? The answer is  ajñas tamasā : because of gross ignorance. One who is in complete darkness cannot know what his past life was or what his next life will be; he is simply interested in his present body. Even though he has a human body, a person in the mode of ignorance and interested only in his present body is like an animal, for an animal, being covered by ignorance, thinks that the ultimate goal of life and happiness is to eat as much as possible. A human being must be educated to understand his past life and how he can endeavor for a better life in the future. There is even a book, called  Bhṛgu-