Mary Douglas, in Purity and Danger describes dirt as "that which has no place." As we know it, dirt is essentially disorder. Dirt offends against order. Eliminating it is not a negative moment, but a positive effort to organise the environment. Rituals of purity and impurity create unity in experience. (Mary Douglas, Purity and Danger) (cf formless)
Powerful taboos and sanctions have been attached in many groups to exogamous unions that threaten their physical sameness. “Purity” is the usual word, carrying with it that strong sense that there is contamination in the mixing of one physical stock with another, with “untouchability” in the Hindu caste system a case in point.
"Dirtiness of any kind seems to us incompatible with civilization." (Freud, Civilization and its Discontents, p.40)