Jewish literature of the period reflects a widespread belief in evil spirits and their ability to afflict human beings. Desolate or waterless places could be associated in ancient imagination with dangerous spiritual beings. Jesus uses this familiar conceptual world in a compact illustration.
The 'house' represents the person from whom the spirit departed. The important feature is that the house is empty. Removal of evil without a corresponding positive transformation leaves the person vulnerable. The number seven commonly conveys completeness or fullness, intensifying the picture of the person's final condition.