Scene one: the young man is out with a friend for an evening in Edinburgh. The company in the pub is good, and the drink is flowing. The duo move to a nightclub, where the merriment increases. At three o'clock in the morning, the two men are larking around on George IV Bridge. They laugh when the young man picks up a traffic cone and dances around with it on his head, like a witch's cap. Then he takes the heavy cone in both hands and throws it.

Scene two: slow motion. The cone flies over the railing, then tumbles through the air. It falls 40 feet to Merchant Street, below.

Scene three: a young Irish woman is chatting cheerfully to her friends outside a pub in Merchant Street. She does not know that her life will change dramatically in one second.

Scene four: impact. The girl lies unconscious and bleeding.

She is paralysed. Welcome to Scotland's capital.

This, of course, is no invented scenario. Kate Flannery's very real injuries included a broken leg, a fractured skull and three broken vertebrae. The damage to the 24-year-old occupational-therapy student's spine was such that doctors feared she would never walk again.

Last week, the man who threw the cone walked free from the High Court in Edinburgh. Andrew Smith, a 30-year-old Australian, who had admitted a charge of culpable and reckless conduct, was ordered to carry out 180 hours of community service.

Should he have gone to prison? There were certainly some mitigating factors. Smith had not intended to injure anyone. The court heard that he had handed himself in after the incident was publicised in the media. His case was undoubtedly helped by the fact that Kate Flannery has made a remarkable recovery. Her limbs, though, are likely to be permanently weakened and will be vulnerable to arthritis.

There are still hard questions to be asked. Although Smith didn't mean to hurt anyone, he certainly didn't check what was happening in the street below. Talking about his own reckless youth last week, Alex James, bass guitarist with the band Blur, confessed that he chucked a television out of a London hotel window. "It landed in Shaftesbury Avenue, but I did check there was nobody coming. I was quite responsible," he said. Smith was totally irresponsible.

So why did the judge not jail him? Lady Dorrian was greatly influenced by the injured woman's plea that the man responsible be spared prison. The victim's forgiveness pre-dated her recovery. "I have forgiven him and moved on," she said. "I decided in the first few days that I wasn't going to be angry with him and was going to focus my energies on getting better. I've been recovering and doing really well, so I just want to look forward now."

Lady Dorrian said of the victim: "She has shown great courage, fortitude and strength of character in the way she has dealt with her injuries. She and her parents have shown remarkable forgiveness." She added that Smith had shown contrition in the aftermath of the incident.

Miss Flannery's magnanimous gesture, and her early decision to focus her energies not on bitter recriminations but on her future, introduced a new and positive dynamic. Genuine forgiveness has major consequences, not just in the personal but also in the political sphere. Remember all those predictions of an unavoidable bloodbath in South Africa? The grim prophecies were turned on their head by Nelson Mandela's forgiveness of his enemies.

Speaking of his visit to Robben Island prison, Richard Holloway, the former Bishop of Edinburgh, said he was choked by Mandela's graciousness. "Those are the conditions that normally produce enraged avengers, whose actions we deplore, yet whose embittered logic we can understand forgiveness flowing from such conditions is impossible to understand. It is the insanity of grace."

I'm not totally convinced that the interests of justice were served by this sentence. Calling the incident a "stupid prank", as Flannery did, was generous, but it was hardly an adequate description of what actually happened. The bottom line is that throwing a traffic cone 40 feet into a street is inexcusably reckless behaviour. I'm with Justice Secretary Kenny MacAskill: "It wisnae me, it was the drink", which could serve as one of Scotia's least appealing mottos, is a flimsy defence. It would be disallowed if offered by a drunk driver who "accidentally" left someone paralysed.

But the forgiveness which transcends embittered logic is awesome and inspiring. This "insanity of grace" has handed a young man a chance to remake his life. I hope he does his community service in a ward for paraplegics, and gives thanks to the open-hearted Irish occupational therapist who might never have walked again.