Initially, the visual impact of the scenes - those towers collapsing with malign majesty - extended our state of fevered astonishment. Even on Wednesday, fresh video footage froze us in this stupefied condition, and denied us our profounder feelings: the first plane disappearing into the side of the tower as cleanly as a posted letter; the couple jumping into the void, hand in hand; a solitary figure falling with a strangely extended arm (was it an umbrella serving as a hopeful parachute?); the rescue workers crawling about at the foot of a vast mountain of rubble.

In our delirium, most of us wanted to talk. We babbled, by email, on the phone, around kitchen tables. We knew there was a greater reckoning ahead, but we could not quite feel it yet. Sheer amazement kept getting in the way.

The reckoning, of course, was with the personal. By Thursday I noticed among friends, and in TV and radio commentaries, a new mood of exhaustion and despair. People spoke of being depressed. No other public event had cut so deeply. The spectacle was over. Now we were hearing from the bereaved. Each individual death is an explosion in itself, wrecking the lives of those nearest. We were beginning to grasp the human cost. This was what it was always really about.

The silent relatives grouped around the entrances to hospitals or wandering the streets with their photographs was a terrible sight. It reminded us of other tragedies, of wars and natural disasters around the world. But Manhattan is one of the most sophisticated cities in the world, and there were some uniquely modern elements to this nightmare that bound us closer to it.

The mobile phone has inserted itself into every crevice of our daily lives. Now, in catastrophe, if there is time enough, it is there in our dying moments. All through Thursday we heard from the bereaved how they took those last calls. Whatever the immediate circumstances, what was striking was what they had in common. A new technology has shown us an ancient, human universal.

A San Francisco husband slept through his wife's call from the World Trade Centre. The tower was burning around her, and she was speaking on her mobile phone. She left her last message to him on the answering machine. A TV station played it to us, while it showed the husband standing there listening. Somehow, he was able to bear hearing it again.We heard her tell him through her sobbing that there was no escape for her. The building was on fire and there was no way down the stairs. She was calling to say goodbye. There was really only one thing for her to say, those three words that all the terrible art, the worst pop songs and movies, the most seductive lies, can somehow never cheapen. I love you.

She said it over and again before the line went dead. And that is what they were all saying down their phones, from the hijacked planes and the burning towers. There is only love, and then oblivion. Love was all they had to set against the hatred of their murderers.

Last words placed in the public domain were once the prerogative of the mighty and venerable - Henry James, Nelson, Goethe - recorded, and perhaps sometimes edited for posterity, by relatives at the bedside. The effect was often consolatory, showing acceptance, or even transcendence in the face of death. They set us an example. But these last words spoken down mobile phones, reported to us by the bereaved, are both more haunting and true.

They compel us to imagine ourselves into that moment. What would we say? Now we know.

Most of us have had no active role to play in these terrible events. We simply watch the television, read the papers, turn on the radio again. Listening to the analysts and pundits is soothing to some extent. Expertise is reassuring. And the derided profession of journalism can rise quite nobly, and with immense resource, to public tragedy.

However, I suspect that in between times, when we are not consuming news, the majority of us are not meditating on recent foreign policy failures, or geopolitical strategy, or the operational range of helicopter gunships.

Instead, we remember what we have seen, and we daydream helplessly. Lately, most of us have inhabited the space between the terrible actuality and these daydreams. Waking before dawn, going about our business during the day, we fantasize ourselves into the events. What if it was me?

This is the nature of empathy, to think oneself into the minds of others. These are the mechanics of compassion: you are under the bedclothes, unable to sleep, and you are crouching in the brushed-steel lavatory at the rear of the plane, whispering a final message to your loved one. There is only that one thing to say, and you say it. All else is pointless. You have very little time before some holy fool, who believes in his place in eternity, kicks in the door, slaps your head and orders you back to your seat. 23C. Here is your seat belt. There is the magazine you were reading before it all began.

The banality of these details might overwhelm you. If you are not already panicking, you are clinging to a shred of hope that the captain, who spoke with such authority as the plane pushed back from the stand, will rise from the floor, his throat uncut, to take the controls...

If the hijackers had been able to imagine themselves into the thoughts and feelings of the passengers, they would have been unable to proceed. It is hard to be cruel once you permit yourself to enter the mind of your victim. Imagining what it is like to be someone other than yourself is at the core of our humanity. It is the essence of compassion, and it is the beginning of morality.

The hijackers used fanatical certainty, misplaced religious faith, and dehumanising hatred to purge themselves of the human instinct for empathy. Among their crimes was a failure of the imagination. As for their victims in the planes and in the towers, in their terror they would not have felt it at the time, but those snatched and anguished assertions of love were their defiance.

© Ian McEwan, 2001