Today, we tend to associate the word “purity” with moral qualities—lack of ill intention and especially sexual virtue—but in many of the societies Douglas discusses, purity has literally to do with the cleanliness of the body. Yet for them no less than for us, the idea of purity goes beyond the physical. Douglas interprets the search for purity as a way to create unity in experience, of affirming accepted things and persons in society and rejecting those that don't belong. Excluding things regarded as unclean is a way of categorizing the things around us and thus affirming a society's worldview.

Dirt is something we often take for granted, but what does it really mean and why do we abhor it? For Douglas, the salient feature of dirt is that it is “matter out of place.” For example, a pair of mud-encrusted boots would not be considered dirty out of doors, but would be considered so indoors. The concept of dirt reflects man's need to categorize the things in the universe.

Douglas believes that the concept of dirt can be a bridge between modern and primitive cultures, since it is a thing both condemn—we for hygienic reasons, and they for religious reasons.