We shall be well advised to recall at the outset that the Hindu society, in common with other societies, was composed of classes and the earliest known are the (1) Brahmans or the priestly class: (2) the Kshatriya, or the military class: (3) the Vaiśya, or the merchant class: and (4) the Sudra, or the artisan and menial class. Particular attention has to be paid to the fact that this was essentially a class system, in which individuals, when qualified, could change their class, and therefore classes did change their personnel. At some time in the history of the Hindus, the priestly class socially detached itself from the rest of the body of people and through a closed-door policy became a caste by itself. The other classes being subject to the law of social division of labour underwent differentiation, some into large, others into very minute groups. The Vaiśya and Sudra classes were the original inchoate plasm, which formed the sources of the numerous castes of to-day. As the military occupation does not very easily lend itself to very minute sub-division, the Kshatriya class could have differentiated into soldiers and administrators. This sub-division of a society is quite natural. But the unnatural thing about these sub-divisions is that they have lost the open door character of the class system and have become self-enclosed units called castes. The question is, were they compelled to close their doors and become endogamous, or did they close them of their own accord? I submit that there is a double line of answer: *Some closed the door: others found it closed against them*. The one is a psychological interpretation and the other is mechanistic, but they are complementary and both are necessary to explain the phenomena of caste formation in its entirety.