French forensic scientist Edmond Locard, working at the world's first police crime laboratory in Lyon, France, formulated what became known as Locard's Exchange Principle: every contact leaves a trace. The principle holds that whenever two objects come into contact, material is exchanged between them — a foundational axiom of forensic science still taught and applied today. Locard established his pioneering forensic laboratory in 1910 and used it to analyse hair, dust, fibres, and skin cells as criminal evidence.

Locard's principle — that no encounter leaves the world unchanged — resonates far beyond forensic science into philosophical and literary thought. The idea that contact always leaves a trace becomes a metaphor for memory, guilt, and inescapable history in modernist and post-modernist writing. Poets exploring trauma, encounter, and the persistence of the past draw, often unconsciously, on this forensic logic. The Lyon laboratory also directly inspired the CSI culture of forensic realism that pervades contemporary crime fiction.