Final answer:
The principle being applied when a sorcerer uses items like hair or nail clippings in magic is the law of contagion, a part of sympathetic magic positing that items in contact maintain a connection and can be used to influence someone.
Step-by-step explanation:
When a sorcerer uses items that gain power from contact, such as using hair and nail clippings to harm an enemy, they are applying the law of contagion, which is one of the two principles of sympathetic magic as defined by James G. Frazer. The law of contagion posits that objects that have been in contact with each other retain a connection, allowing a practitioner to affect someone through their personal items. This concept is part of the broader framework of witchcraft and sorcery practices, which manipulate mana, a supernatural force that anthropologists recognize in many religious traditions, and are influenced by cause and effect reasoning strategies as described by Plato.