Born in... (SB cantos 5 - 12)
Srimad-Bhagavatam
SB Canto 5
My dear boy, all of us are bound by the Vedic injunctions to the divisions of varṇāśrama according to our qualities and work. These divisions are difficult to avoid because they are scientifically arranged. We must therefore carry out our duties of varṇāśrama-dharma, like bulls obliged to move according to the direction of a driver pulling on ropes knotted to their noses.
In this verse, the words tantyāṁ guṇa-karma-dāmabhiḥ are very important. We each get a body according to our association with the guṇas, the qualities or modes of material nature, and we act accordingly. As stated in Bhagavad-gītā, the four orders of the social system—namely brāhmaṇa, kṣatriya, vaiśya and śūdra—are arranged according to guṇa and karma, their qualities and work. There is some controversy about this, however, because some say that since one receives a body according to the guṇa and karma of his past life, it is one's birth that determines his social status. Others say, however, that one's birth according to the guṇa and karma of his past life is not the essential consideration, since one can change his guṇa and karma even in this life. Thus they say that the four divisions of the social order—brāhmaṇa, kṣatriya, vaiśya and śūdra—should be arranged according to the guṇa and karma of this life. This version is confirmed in Śrīmad-Bhāgavatam by Nārada Muni. While instructing Mahārāja Yudhiṣṭhira about the symptoms of guṇa and karma, Nārada Muni said that these symptoms must govern the division of society. In other words, if a person born in the family of a brāhmaṇa has the symptoms of a śūdra, he should be designated as a śūdra. Similarly, if a śūdra has brahminical qualities, he should be designated a brāhmaṇa.