Out of eighty four lakhs of varieties or species of life there are only four lakhs varieties of human being. Out of these varieties of human being those who can acquire the the qualities of goodness out of the three modes of nature, can also realize the proper self which is not the product of material world.
The Vedic culture determines the standard of social upliftment in terms of the degree of self-realization and as such the social division was estimated accordingly in terms of goodness (intelligence and higher learning), passion (martial spirit to lord it over the world), passion-cum-ignorance (the spirit of productivity) and ignorance (the spirit of passive acceptance of being controlled by the laws of material nature).