This is confirmed by Krishna, God, the Supreme Person: One who is completely absorbed in devotional service, without ever faltering, transcends the three attributes and modes of influence of the material nature and thus attains the spiritual level.
Unless one is fully absorbed in the service of the Lord, one remains exposed to the defilement of the three attributes of the material nature, such that one must endure the sufferings associated with misfortune or a mixture of happiness and misfortune.
A life of happiness, misfortune, or mixed feelings allows one to determine the proportion of virtuous and impious acts in past and future lives. It is not very difficult to know one's past and future, for time reflects the contamination of the three attributes of the material nature.
Currently, most incarnate spiritual entities identify with their present body, the result of their past virtuous or impious acts, and remain incapable of knowing their previous or future lives. Man gives himself over to sin because he is unaware of the deeds of his past life that have brought him to his present condition, in a physical body exposed to the three forms of suffering. He is seized by a desperate need for material pleasures, does not hesitate to sin, and commits reprehensible acts solely to satisfy his senses. All of this is harmful, for because of his sins, he will have to receive another body in which he will suffer just as he suffers now from the consequences of his past transgressions.
In this world, we encounter three forms of suffering: those originating from the body and mind, those caused by other living entities, and those that originate from the material world itself—hurricanes, violent winds, torrential rains, extreme cold, drought, etc.—under the influence of beings from the higher planets, who govern the various functions of the material world.
It is crucial to understand that a person deprived of spiritual knowledge constantly acts in ignorance of their past actions or previous lives, their current actions, and their future suffering; they are plunged into darkness. Therefore, we must not remain in darkness, but strive to attain the transcendental light. This light is the spiritual knowledge disseminated by Krishna, God, which can be known once one has reached the level of virtue, or when one transcends virtue by embracing the practice of devotional service offered to the spiritual master, the true intimate servant of God and the Supreme Lord Krishna.
The meaning and scope of spiritual knowledge are revealed in their fullness, all at once, only to great souls endowed with unwavering faith in God. Depending on our relationship with the attributes of our material nature (virtue, passion, and ignorance), we acquire a particular type of body.
Anyone plunged into total darkness cannot know what their past life was like, nor what their next life will be like. They are interested only in their present body. Even if they possess a human form, someone who is under the influence of ignorance and concerned only with their physical body is scarcely better than an animal. Indeed, the animal imprisoned by ignorance believes that the greatest happiness and the ultimate goal of life consists in eating as much as possible.