Bhagavad Gita 18.41ब्राह्मणक्षत्रियविशां शूद्राणां च परंतप।कर्माणि प्रविभक्तानि स्वभावप्रभवैर्गुणैः ॥

Brahmanas, Kshatriyas and Vaisyas are qualified to practise the Vedic rites. The members of the fourth class, O Arjuna, have no claim to these rites, for their profession is to serve the members of the first three. They are not allowed to study the Vedas or perform Yajnas. There is organisation of mankind into the four castes and each mans life is divided into four stages, according to the nature of the Gunas and the degree of the growth or evolution. I will now explain the specific duties of these castes according to the alities by means of which, freeing themselves from the grip of birth and death, they can attain Selfrealisation or knowledge of the Self. Passion (Rajas) slightly mingled with Tamas causes the growth of the merchant caste (the Vaisya). Rajas mixed with Tamas is the cause of the appearance of the Sudra.The one human race has been divided into four castes based upon the three alities. The duties are allotted to each according to the alities born of Nature.Nature (Svabhava) is Isvaras Prakriti (Maya) constituted of the three alities -- Sattva, Rajas and Tamas.The Brahmanas nature is Sattva. So he is serene. The nature of the Kshatriya is Rajas and Sattva. Sattva in his case is subordinate to Rajas. Rajas predominates. Therefore he possesses lordliness. The nature of the Vaisya is Rajas and Tamas. Tamas is subordinate to Rajas