Those who have come to the platform of human civilization should be divided into a society of brahmanas, ksatriyas, vaisyas and sudras. The brahmanas should follow the instructions of the Supreme Personality of Godhead as stated in Bhagavad-gita and other Vedic literatures. The criterion must be guna and karma. In other words, one should acquire the qualities of a brahmana, ksatriya, vaisya or sudra and act accordingly. This is the civilization accepted by the Aryans. Why do they accept it? They accept it because they are very much eager to satisfy Krsna. This is perfect civilization.
(From Srila Prabhupada's purport to SB 6.16.43)