Law of contagion
The belief that things once in contact stay linked, so that hair, nails or a worn garment can stand in for a person.
Also: contagious magic, once in contact, always in contact
Frazer's second law of sympathetic magic says that objects which have touched remain joined after they part. A lock of hair, a nail paring, a footprint or a piece of clothing is believed to keep a bond with its owner. For this reason, many peoples have taken care over what is discarded, and some rites are said to work through such remnants. The same logic explains why relics of saints are valued, and why some people feel a gift from a loved one carries something of them. Psychologist Paul Rozin has studied the idea experimentally and calls it a widespread intuition in modern societies too.