On September 11 2001, I was in London, leading training with fellow senior mediators. At some point in the afternoon, David Richbell, a senior colleague, came into the room. He described two incidents in the United States and said that the twin towers of the World Trade Center had collapsed.
As with everyone who heard that news that day, there was a stunned silence. We gathered ourselves and watched events on a nearby TV screen.
We then had a choice to make. To continue with our training programme - or to go home and reflect. As course leader, it fell to me to say something. I felt that we needed to go on for at least two reasons: one, not to let terrorism stop us from going about our business and, two, now as never before, the world needed the skills of peacemakers. We went ahead.
On July 7 2005, I was also in London, leading training for a group of aspiring mediators. The impact of the tube bombing that day was even more dramatic of course. People did not know if loved ones were affected - or how widespread the acts of violence might be. The mobile phone network went down for a while. We could not make contact with the outside world. After some time, most of us were reassured that there was no direct impact on the group.
But, again, we had to decide whether to continue. The answer was the same. We needed to go on. We did.
This Sunday, in various towns and cities around the world, the events of 911 are being remembered with walks for peace. Organised under the auspices of William Ury's Abraham's Path initiative ( www.abrahampath.org), the walks seek to bring people of all faiths together for a period of time, to show solidarity and commitment to peaceful co-existence. There is one in Edinburgh.
Walking together, side by side, is the only path we can take if we wish to secure a peaceful future.