This regulative principle is applicable to all varṇas and āśramas, the castes and occupations of life. There are four varṇas, namely, the brāhmaṇas (priests and intellectuals), the kṣatriyas (warriors and statesmen), the vaiśyas (businessmen and farmers) and the śūdras (laborers and servants). There are also four standard āśramas, namely brahmacarya (student life), gṛhastha (householder), vānaprastha (retired) and sannyāsa (renounced). The regulative principles are not only for the brahmacārīs (celibate students) to follow, but are applicable for all. It doesn't matter whether one is a beginner—a brahmacārī—or if one is very advanced—a sannyāsī. The principle of remembering the Supreme Personality of Godhead constantly and not forgetting Him at any moment is meant to be followed by everyone without fail.
If this injunction is followed, then all other rules and regulations will automatically fall into line. All other rules and regulations should be treated as assistants or servants to this one basic principle. The injunctions of rules and regulations and the resultant reactions are mentioned in the Eleventh Canto of Śrīmad-Bhāgavatam, 5th Chapter, 1st and 2nd verses. Camasa Muni, one of the nine sages who came to instruct King Nimi, addressed the King and said, "The four social orders, namely the brāhmaṇas, the kṣatriyas, the vaiśyas, and the śūdras, have come out of the different parts of the universal form of the Supreme Lord as follows: the brāhmaṇas have come out from the head, the kṣatriyas have come out from the arms, the vaiśyas have come out from the waist, and the śūdras have come out from the legs. Similarly, the sannyāsīs have come out from the head, the vānaprasthas from the arms, the gṛhasthas from the waist, and the brahmacārīs from the legs."
These different orders of society and grades of spiritual advancement are conceived in terms of qualification. It is confirmed in the Bhagavad-gītā that the four social orders and the four spiritual orders are created by the Lord Himself, in terms of different individual qualities. As the different parts of the body have different types of activities, so the social orders and spiritual orders also have different types of activities in terms of qualification and position. The target of these activities, however, is always the Supreme Personality of Godhead. As confirmed in the Bhagavad-gītā, "He is the supreme enjoyer." So whether one is a brāhmaṇa or a śūdra, he has to satisfy the Supreme Lord by his activities. This is also confirmed in the Śrīmad-Bhāgavatam by a verse which reads: "Everyone must be engaged in his particular duty, but the perfection of such work should be tested by how far the Lord is satisfied with such activities." The injunction herein is that one has to act according to his position, and by such activities one must either satisfy the Supreme Personality or else fall down from his position.
For example, a brāhmaṇa who is born out of the head of the Lord has as his business to preach the transcendental Vedic sounds, or śabda-brahman. Because the brāhmaṇa is the head, he has to preach the transcendental sound, and he also has to eat on behalf of the Supreme Lord. According to Vedic injunctions, when a brāhmaṇa eats it is to be understood that the Personality of Godhead is eating through him. It is not, however, that the brāhmaṇa should simply eat on behalf of the Lord and not preach the message of the Bhagavad-gītā to the world. Actually, one who preaches the message of the Gītā is very dear to Kṛṣṇa, as is confirmed in the Gītā itself. Such a preacher is factually a brāhmaṇa and thus by feeding him one feeds the Supreme Lord directly.
Similarly, the kṣatriya has to protect people from the onslaughts of māyā. That is his duty. For example, as soon as Mahārāj Parīkṣit saw that a black man was attempting to kill a cow, he immediately took his sword, wanting to kill the black man, whose name was Kali.* That is a kṣatriya's duty. Violence is required in order to give protection. In the Bhagavad-gītā, Lord Kṛṣṇa directly gave His order to Arjuna to commit violence on the battlefield of Kurukṣetra, just to give protection to the people in general.