In emphasizing an ongoing procedure or set of habits for the purpose of maintenance and prevention, the concept of cleanliness differs from , which is a physical, moral, or ritual state of freedom from pollutants. Whereas purity is usually a quality of an individual or substance, cleanliness has a social dimension and direction or implies a system of interactions. "Cleanliness," observed Jacob Burckhardt, "is indispensable to our modern notion of social perfection." A household or workplace may be said to exhibit cleanliness, but not ordinarily purity; cleanliness also would be a characteristic of the people who maintain cleanness or prevent dirtying.