Verse 18·41

ब्राह्मणक्षत्रियविशां शूद्राणां च परन्तप ।कर्माणि प्रविभक्तानि स्वभावप्रभवैर्गुणैः ॥

brāhmaṇakṣatriyaviśāṃ śūdrāṇāṃ ca parantapakarmāṇi pravibhaktāni svabhāvaprabhavairguṇaiḥ

These are not fundamental divisions, but stages of self-development in our manhood. The human being starts with a sufficient load of ignorance and inertia; his first state is one of rude toil enforced on his animal indolence by the needs of the body, by the impulsion of life, by necessity of Nature and, beyond a certain point of need, by some form of direct or indirect compulsion which society lays upon him, and those who are still governed by this tamas are the Shudras, the serfs of society who give it their toil and can contribute nothing or very little else in comparison with more developed men to its manifold play of life. By kinetic action man develops the rajasic guna in him and we get a second type of man who is driven by a constant instinct for useful creation, production, having, acquisition, holding and enjoying, the middle economic and vital man, the Vaishya. At a higher elevation of the rajasic or kinetic quality of our one common nature we get the active man with a more dominant will, with bolder ambitions, with the instinct to act, battle, and enforce his will, at the strongest to lead, command, rule, carry masses of men in his orbit, the fighter, leader, ruler, prince, king, Kshatriya. And where the sattwic mind predominates, we get the Brahmin, the man with a turn for knowledge, who brings thought, reflection, the seeking for truth and an intelligent or at the highest a spiritual rule into life and illumines by it his conception and mode of existence.
There is always in human nature something of all these four personalities developed or undeveloped, wide or narrow, suppressed or rising to the surface, but in most men one or the other tends to predominate and seems to take up sometimes the whole space of action in the nature.