Puṣṭa Kṛṣṇa: How does the spirit soul of a beast enter into the form of a human being?
Prabhupāda: Just like a thief in the prison house. How he becomes liberated? When his term of suffering in the prison house is finished, then he is again free man. And again if he is criminal, he is put into jail. So human form of life is meant for understanding, as I am explaining, what is the problem of my life. I do not wish to die; I am put to death. I do not wish to become old man; I am obliged to become old man. Janma-mṛtyu-jarā-vyādhi-duḥkha-doṣānudarśanam (BG 13.9). So he... Just like the same example, a thief. When he is free, if he thinks, ponders, that "Why I was put into this miserable condition of six month prison life? It was so botheration," then he becomes actually human being. So similarly, the human being has got advanced power of deliberation. If he thinks that "Why I am put into this miserable condition?" Everyone has to admit that he is in miserable condition. He is trying to become happy, but there is no happiness. So how that happiness can be achieved? That chance is in the human being. But if we receive, by the mercy of the material nature, a human being and we do not utilize it properly, if we misuse this benediction as cats and dogs or other animals, then we have to accept again the animal form, and when the term is finished... It takes long, long duration of time because there is evolutionary process. So again you'll come to this human form of life, when the term is finished. Exactly the same example: A thief, when he has finished his term of imprisonment, he's again a free man. But again he commits criminality; again he goes to the jail. So there is cycle of birth and death. If we utilize our human form of life properly, then we stop the cycle of birth and death. And if we do not use this human form of life properly, again we go to that cycle of birth and death.