Written in 1966, Douglas was interested in the social and cultural meaning of dirt. She concluded there is not universal notion of 'dirt' - nothing is objectively dirty across culture and history.
We have customs and rituals that keep our culturally specific idea of dirt within certain boundaries. Showering, using napkins at the table, taking off shoes before walking into the house.
In our Western scientific worldview largely influenced by germ theory and medical advice, we define many of our ideas of 'dirt' in the language of viruses, bacteria, disease, and infection.
Cleaning is an act of constructing the world as we imagine it. We draw dividing lines between things. We build type of what belongs where. Things are only dirty when they cross our pre-defined boundaries. They are “matter out of place”.