The Vedānta-sūtra explains when the question is that what is the original cause of everything . . . what is Brahman? What is the Absolute Truth? Athāto brahma jijñāsā: "Now this human form of life is meant for inquiring about the Absolute Truth." What is that Absolute Truth? The next answer is, janmādy asya yataḥ (SB 1.1.1), means "The Absolute Truth is that from whom everything emanates," or "The Absolute Truth is the original cause of all causes." This is the . . . so this knowledge, anyone who has knowledge, not only this knowledge, absolute knowledge as well as relative knowledge, such class of men is called the brāhmaṇas, the most intelligent class.
The next class is called the kṣatriya. Just like when somebody comes to attack you, first of all your brain gives you dictation, "Now this man is coming to attack you. You spread your hand." So immediately my hand spreads and I want to protect myself. So these are called kṣatriya class, or the armies, from "arm." So next-intelligent class is the government class, administrator class. Then next-intelligent class is third class. First class, brāhmaṇas, second class, the kṣatriyas, and the third class, the vaiśyas, who maintain the society for economic condition, development of economic condition, because we require things to consume to maintain this body. So these are called mercantile class, and the ordinary man who is neither brāhmaṇa nor kṣatriya nor vaiśya, he is called śūdra.