As stated in the Eighteenth Chapter of Bhagavad-gītā, sva-karmaṇā tam abhyarcya: one has to worship the Supreme Personality of Godhead by one's occupational duties. This necessitates accepting the principle of four varṇas and four āśramas. Pṛthu Mahārāja therefore says, guṇaiḥ sva-karmabhiḥ. This phrase is explained in Bhagavad-gītā. Cātur-varṇyaṁ mayā sṛṣṭaṁ guṇa-karma-vibhāgaśaḥ: "The four castes (the brāhmaṇas, kṣatriyas, vaiśyas and śūdras) are created by the Supreme Personality of Godhead according to the material modes of nature and the particular duties discharged in those modes." A person who is situated in the mode of goodness is certainly more intelligent than others. Therefore he can practice the brahminical activities—namely speaking the truth, controlling the senses, controlling the mind, remaining always clean, practicing tolerance, having full knowledge about one's self-identity, and understanding devotional service. In this way, if he engages himself in the loving service of the Lord as an actual brāhmaṇa, his aim to achieve the final interest of life is attained. Similarly, the kṣatriya's duties are to give protection to the citizens, to give all his possessions in charity, to be strictly Vedic in the management of state affairs and to be unafraid to fight whenever there is an attack by enemies. In this way, a kṣatriya can satisfy the Supreme Personality of Godhead by his occupational duties. Similarly, a vaiśya can satisfy the Supreme Godhead by properly executing his occupational duties—engaging himself in producing foodstuffs, giving protection to cows, and trading if necessary when there is an excess of agricultural production. Similarly, because śūdras do not have ample intelligence, they should simply engage as workers to serve the higher statuses of social life. Everyone's aim should be to satisfy the Supreme Personality of Godhead by engaging his mind in thinking always of Kṛṣṇa, his words in always offering prayers to the Lord or preaching about the glories of the Lord, and his body in executing the service required to satisfy the Lord. As there are four divisions within our body—the head, the arms, the belly and the legs—similarly, human society, taken as a whole, is divided into four classes of men according to their material qualities and occupational duties. Thus the brahminical or intelligent men have to execute the duty of the head, the kṣatriyas must fulfill the duty of the arms, the vaiśya class must fulfill the duty of the belly, and the śūdras must fulfill the duty of the legs. In executing the prescribed duties of life, no one is higher or lower; there are such divisions as "higher" and "lower," but since there is actually a common interest—to satisfy the Supreme Personality of Godhead—there are no distinctions between them.
The question may be raised that since the Lord is supposed to be worshiped by great demigods like Lord Brahmā, Lord Śiva and others, how can an ordinary human being on this planet serve Him? This is clearly explained by Pṛthu Mahārāja by the use of the word yathādhikāra, "according to one's ability." If one sincerely executes his occupational duty, that will be sufficient. One does not need to become like Lord Brahmā, Lord Śiva, Indra, Lord Caitanya or Rāmānujācārya, whose capabilities are certainly far above ours. Even a śūdra, who is in the lowest stage of life according to the material qualities, can achieve the same success. Anyone can become successful in devotional service provided he displays no duplicity. It is explained here that one must be very frank and open-minded (amāyinaḥ). To be situated in a lower status of life is not a disqualification for success in devotional service. The only qualification is that whether one is a brāhmaṇa, kṣatriya, vaiśya or śūdra, he must be open, frank and free from reservations. Then, by performing his particular occupational duty under the guidance of a proper spiritual master, he can achieve the highest success in life. As confirmed by the Lord Himself, striyo vaiśyās tathā śūdrās te 'pi yānti parāṁ gatim (BG 9.32). It does not matter what one is, whether a brāhmaṇa, kṣatriya, vaiśya, śūdra or a degraded woman. If one engages himself seriously in devotional service, working with body, mind and intelligence, he is sure to be successful in going back home, back to Godhead. The Lord's lotus feet are described here as kāma-dughāṅghri-paṅkajam because they have all power to fulfill the desires of everyone. A devotee is happy even in this life because although in material existence we have many needs, all his material needs are satisfied, and when he at last quits his body, he goes back home, back to Godhead, without a doubt.