Srimad Bhagvata Mahapurana: Book 10: Chapter 24: Verses 15-30
What have created beings-that follow each the course of its own action in this world-to do with Indra, who is incapable of altering the course of actions performed by men according to their individual nature ? Indeed every created being is a slave to its nature and follows its own natural disposition. The whole of this creation including gods, demons and human beings has its being in nature. By force of Karma a soul takes (diverse) corporeal forms, high and low, and quits them. Karma alone appears in the form of an enemy, a friend or a neutral and Karma alone is our preceptor and almighty Lord. Therefore, sticking to the Varna (grade of society) and Asrama (stage in life) determined by one's own nature and performing one's own duty, one should duly worship Karma alone. In fact, that alone is one's deity by which one lives happily (in this world). Depending on a particular deity, he who worships another does not derive happiness from the latter any more than an unchaste woman who loves a paramour. A Brahmana should live by (the teaching and exposition of) the Vedas; a Ksatriya, by protecting the land; a Vaisya, by (what is known as) Varta; while a Sudra should live by service (rendered) to the (aforesaid) twice-born classes. Varta is said to be of four kinds, viz., agriculture, commerce and rearing the bovine race, usury being the fourth. Out of these (four) the cow has ever been our (only) means of subsistence. (The qualities of) Sattva, Rajas and Tamas are severally the cause of the continuance, appearance and dissolution (of the universe); it is through (the quality of) Rajas that by mutual union (of the male and female) is brought forth the heterogenous universe. Impelled by Rajas, clouds shower water all round. It is through such water that living beings achieve their purpose (of getting their food etc.).