 He believed that the caste system originated in the practise of endogamy and that it spread through imitation by other groups.He wrote that initially, Brahmins, Kshatriyas, Vaishyas and Shudras existed as classes whose choice of occupation was not restricted by birth and in which exogamy was prevalent.Brahmins then began to practise endogamy and enclosed themselves, hence Ambedkar defines caste as "enclosed class".