“Every time two people come into contact with one another, an exchange takes place. A trace is left. Spoken or unspoken; for better or for worse; we smile or we scowl; we encourage or we ignore; we hold out a hand of friendship or we withdraw it; we cross the road to help someone in trouble, or we turn around and walk in the opposite direction; we bless or we curse; we love or we hate.” – John Sutherland

Put simply, every time two objects come into contact with one another, an exchange takes place – and a trace is left.

Imagine the scene:

A thief climbs over a fence into a suburban back garden, leaving his footprints in the flowerbed. He makes his silent approach to the rear of the house and breaks a window, cutting himself in the process and leaving drops of blood on the sill. As he climbs through the window, he catches his clothing on the broken edges of the glass, leaving fibres behind and, once inside, he leaves his fingerprints on the surfaces he touches.

Each of these things – the footprints, the blood, the fibres, the fingerprints – are traces of the suspect, left at the scene. And the first police officer to arrive ought to be able to preserve those traces as evidence.

But that is not all.

A colleague of the first officer spots the burglar further down the road, detains him, and finds on him traces of the scene that he has carried with him: soil from the flowerbed on the soles of his shoes; microscopic fragments of glass spread across his clothing; the items he has stolen hidden in the bag carried on his back.

Traces from the suspect, left at the scene. Traces from the scene, carried by the suspect. Put those two sets of things together, and you are well on your way to solving the case.

Because every contact leaves a trace.

But there is an application of Locard’s Principle that goes way beyond simple forensics. Because, significant though it might be as a scientific idea, it becomes even more powerful when applied to the relationships between human beings.

Every time two people come into contact with one another, an exchange takes place. A trace is left. Spoken or unspoken; for better or for worse; we smile or we scowl; we encourage or we ignore; we hold out a hand of friendship or we withdraw it; we cross the road to help someone in trouble, or we turn around and walk in the opposite direction; we bless or we curse; we love or we hate.

And every single contact between us leaves a trace.