SB 6.1.49
yathajñas tamasa yukta
upaste vyaktam eva hi
na veda purvam aparam
nasta-janma-smrtis tatha
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
As a sleeping person acts according to the body manifested in his dreams and accepts it to be himself, so one identifies with his present body, which he acquired because of his past religious or irreligious actions, and is unable to know his past or future lives.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
A man engages in sinful activities because he does not know what he did in his past life to get his present materially conditioned body, which is subjected to the threefold miseries. As stated by Rsabhadeva in Srimad-Bhagavatam (5.5.4), nunam pramattah kurute vikarma: a human being who is mad after sense gratification does not hesitate to act sinfully. Yad indriya-pritaya aprnoti: he performs sinful actions simply for sense gratification. Na sadhu manye: this is not good. Yata atmano ’yam asann api klesada asa dehah: because of such sinful actions, one receives another body in which to suffer as he is suffering in his present body because of his past sinful activities.
It should be understood that a person who does not have Vedic knowledge always acts in ignorance of what he has done in the past, what he is doing at the present and how he will suffer in the future. He is completely in darkness. Therefore the Vedic injunction is, tamasi ma: “Don’t remain in darkness.” Jyotir gama: “Try to go to the light.” The light or illumination is Vedic knowledge, which one can understand when he is elevated to the mode of goodness or when he transcends the mode of goodness by engaging in devotional service to the spiritual master and the Supreme Lord. This is described in the Svetasvatara Upanisad (6.23):
yasya deve para bhaktir
yatha deve tatha gurau
tasyaite kathita hy arthah
prakasante mahatmanah
“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-vijñanartham sa gurum evabhigacchet: 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 brahmana. Such a brahmana knows past, present and future because he consults the Vedic literature and sees through the eyes of sastra (sastra-caksuh). He can understand what his past life was, why be is in the present body, and how he can obtain liberation from the clutches of maya 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, Paramatma. As stated in the previous verse:
manasaiva pure devah
purva-rupam vipasyati
anumimamsate ’purvam
manasa bhagavan ajah
Everything depends on bhagavan, or ajah, the unborn. Why doesn’t one please Bhagavan to receive a better body? The answer is ajñas tamasa: 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 Bhrgu-samhita, which reveals information about one’s past, present and future lives according to astrological calculations. Somehow or other one must be enlightened about his past, present and future. One who is interested only in his present body and who tries to enjoy his senses to the fullest extent is understood to be engrossed in the mode of ignorance. His future is very, very dark. Indeed, the future is always dark for one who is grossly covered by ignorance. Especially in this age, human society is covered by the mode of ignorance, and therefore everyone thinks his present body to be everything, without consideration of the past or future.