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 Ṛṣabhadeva in Śrīmad-Bhāgavatam (5.5.4), nūnaṁ pramattaḥ kurute vikarma: a human being who is mad after sense gratification does not hesitate to act sinfully. Yad indriya-prītaya āpṛṇoti: he performs sinful actions simply for sense gratification. Na sādhu manye: this is not good. Yata ātmano 'yam asann api kleśada āsa dehaḥ: 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 mā:"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 Śvetāśvatara Upaniṣad (6.23):

"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." (Śvetāśvatara Upaniṣad 6.23)"No one can understand Kṛṣṇa as He is by the blunt material senses. But He reveals Himself to the devotees, being pleased with them for their transcendental loving service unto Him." (Bhakti-rasāmṛta-sindhu 1.2.234)"One can understand the Supreme Personality as He is only by devotional service. And when one is in full consciousness of the Supreme Lord by such devotion, he can enter into the kingdom of God." (Bg. 18.55)

These are Vedic instructions. One must have full faith in the words of the spiritual master and similar faith in the Supreme Personality of Godhead. Then the real knowledge of ātmā and Paramātmā and the distinction between matter and spirit will be automatically revealed. This ātma-tattva, or spiritual knowledge, will be revealed within the core of a devotee's heart because of his having taken shelter of the lotus feet of a mahājana such as Prahlāda Mahārāja.6.23]

"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: [MU tad-vijñānārthaṁ sa gurum evābhigacchet
samit-pāṇiḥ śrotriyaṁ brahma-niṣṭham

"To understand these things properly, one must humbly approach, with firewood in hand, a spiritual master who is learned in the Vedas and firmly devoted to the Absolute Truth."
[Muṇḍaka Upaniṣad 1.2.12]

1.2.12] 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 be 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:

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-saṁhitā, 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.