What Do We Really Leave Behind?

A French criminologist named Edmond Locard formulated what is known as Locard’s Exchange Principle. In simple terms, he argued that whenever someone passes through a room, they leave something behind and carry something away. Modern forensic science still uses this idea today. 

A few weeks ago, I became a grandfather again.

Holding my wee granddaughter in my arms prompted me to reflect on what truly matters and what it means to be human. Moments like this, alongside preparing for a talk I’m giving this week, made me think about the impact we have on each other and what we actually leave behind.

Every second, around four babies are born somewhere in the world while roughly two people die. By the time you finish reading this email, hundreds of new little humans will have arrived, while others will have quietly slipped from this life. Life just keeps moving. Generations replace one another. Stories, faces, dreams, laughter, grief, fear, and hope are all woven together.

The Earth’s population at the time of Julius Caesar was probably around 300 million people. Today, the world population grows by roughly 70–80 million people every year. Yet here is the thing that really blows my mind: out of all the billions of people who have ever lived throughout human history, there has never been another you.

Not just physically, but you.

Your personality, your humour, your imagination, your wounds, your story, your way of seeing the world. There has never been anyone quite like you before, and there never will be again. And that uniqueness matters more than we perhaps realise.

Because I sometimes wonder if Locard’s principle is also true at a much deeper human level. Every person who passes through our lives leaves something behind and takes something away. A conversation. A kindness. A wound. An encouragement. A memory. A possibility.

Most of these things cannot be measured. They do not appear in reports, spreadsheets, or census data, yet they often become the very things that shape our lives most deeply.

You may never fully realise what your life has meant to someone else. A passing conversation you barely remember may still be carrying someone years later. A small act of kindness may have altered the direction of another person’s story without you ever knowing it.

As I recently held my granddaughter, I found myself thinking that perhaps what endures most is not our achievements, titles, or productivity, but our presence. Love. Courage. Kindness. Hope.

For better or worse, we unknowingly leave something and take something away. Most of this “something” cannot be seen, heard, or numbered. It will never show up in a census. But nothing counts without it.