Edmond Locard’s “Exchange Principle”

Black and white photo of Edmund Locard in an early crime lab with chemicals.
Edmond Locard in an early crime lab.

Edmond Locard (1877-1966) was a French criminologist and is considered the “father of forensic science.”

His most famous legacy is probably “Locard’s Exchange Principle,” which is the basic notion that “Every contact leaves a trace.”

More specifically, whenever two objects come into contact, there is always a transfer of material. This principle is a foundation for the forensic science of tracing interactions with a crime scene.

But this simple principle also opens some philosophical and phenomenological inquiries about the nature and implications of: residue, impact, influence, responsibility, and so on.

If we were to really follow the paths of exchange — a kind of “contact tracing” between the “scene of the crime” and all the actors who contributed to/crossed it, how far could we go? What could we find?

What kinds of “residue” can we track, and not track?

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