Which principle states that every contact leaves a trace?

The Principle of Exchange is a fundamental concept in forensic science, particularly associated with the work of Edward Locard. It asserts that whenever two objects come into contact with each other, there is a mutual exchange of materials between them. This means that a perpetrator at a crime scene will invariably leave behind traces of their presence, whether it be fibers, hair, or other materials, and, conversely, they may also take away traces from the scene.

This principle is crucial in forensic investigations because it reinforces the idea that physical evidence can connect a suspect to a crime scene. It highlights the importance of collecting and analyzing evidence, as even minute traces can potentially link individuals to criminal activities.

In contrast, the other principles mentioned do not specifically encapsulate the concept of material exchange upon contact. The Principle of Contact is not widely recognized as a formal principle in forensic science, the Principle of Identification focuses on the methods used to positively identify individuals or objects, and the Principle of Continuity relates to maintaining the chain of evidence in an investigation. Each of these plays a different role in forensic analysis and does not capture the essence of the mutual exchange of evidence that occurs during contact.