We must also mention those individuals who are without caste, the untouchables, the impure.
The first clarification to be made is that this type of person should not be imagined as “below” the other three castes, because here it is not a matter of hierarchical relation. What is in question is ‘impurity,’ and impurity is not a matter of low caste but of no caste, so that the untouchable is not so much at the bottom of the latter as they are outside of it.
In other words, the child of two Shudra parents is pure, and is in this sense just as pure as the child of two Bhramana parents. There is no discrepancy in his heredity and so he will suffer from the least amount of internal dissonance.
Impurity is a result, not of low caste, but of mingled caste:
“Illicit mingling of castes, marriages contrary to the rules and the omission of prescribed rites are the origin of the impure classes.”
It is important to understand this because it allows one to better appreciate the purpose of caste, which is not to devalue certain groups but to organize individuals in such a way that they can cooperate and relate. It is no degradation to be a part of the lower caste. Impurity, however, is something else.
Impurity is a discrepancy of tendencies within oneself, and in caste theory it results from a discrepancy of caste between the parents. When a Shudra marries a Vaishya, the children will be impure. However, these children will not be considered as impure as the children who would result from the union of a Bhramana and a Shudra, because in the latter case the inner discrepancy, which has to potential to create the maximum of inner dissonance, is most extreme. It is this latter case that is said to result in the chandala, the “untouchable” who is responsible for the disposal of corpses.
We should pause to remark that corpses must be disposed of, so that from a social point of view the untouchable is still necessary and even though the discrepancy of inner tendencies lends to this group a propensity toward transgression, the theory of caste is capable of incorporating such a man into the order of social life in a way that allows him to participate and develop. The dignity of the lower castes and the untouchables themselves is not stolen from them. Their humanity has not, therefore, ever been denied them or their condition as degraded as, for example, the African slaves used in the antebellum United States.
 Manava Dharma Shastra, X. 24.