Remember these people:
Aysha Frade, a school administrator with a Spanish mother and a Cypriot father, on her way to pick up her children from school.
Kurt Cochran, an American from West Bountiful in Utah, on a tour of Europe with his wife Melissa to celebrate their 25th wedding anniversary.
Leslie Rhodes, aged 75, who died of his injuries late on Thursday night.
Keith Palmer, a police officer with 15 years' experience, a member of the Metropolitan police parliamentary and diplomatic protection command.
Four victims of a callous murderer whose name need not concern us. (Although now that we know that he was born in Kent as Adrian Ajao, I look forward to the apologies from all the racist bigots who claimed that the attack was in some way related to immigration.)
How unlucky his victims were to be at Westminster on Wednesday afternoon -- three of them walking across Westminster Bridge, the fourth doing his job at the entrance to the houses of parliament.
And as we mourn all victims of politically-motivated killings, let us also remember Lee Rigby, the off-duty soldier who was murdered in 2013, and Jo Cox, the MP who was killed last June and whose husband Brendan has been a role model ever since as we struggle to find the right words in response to such cruelty.
After the Westminster attacks on Wednesday, he said: 'The person who did this wants us to be fearful and divided. Let's show them that we are neither.'
In Paris 16 months ago, 130 people died when gunmen opened fire in a series of coordinated attacks. In Brussels, exactly a year ago, 32 people were killed. In Nice, last July, 86 died when a lorry ploughed through Bastille Day crowds on the Promenade des Anglais. And in Berlin last December, 12 died in a similar attack on a Christmas market.
So we may be forgiven for thinking that London got off lightly. We knew the city was not immune, we knew that the security services believed an attack was 'highly likely'. It was a question of when, not if.
Why did London get off lightly? It is tempting to say that we were lucky, but luck was only part of it. The attacker was armed with only knives. No gun -- because it's not easy to get hold of guns in a country with strict laws about the ownership of firearms.
He couldn't get into the Palace of Westminster because it is extremely well-fortified. Those hideous black security barriers are there for a reason. If he could have, I'm sure he would have loved to kill some MPs. PC Palmer was in his way, and gave his life to defend them.
Let us not forget: in 1979, the senior Conservative MP Airey Neave was murdered when Irish republican bombers placed an explosive device beneath his car while it was in the House of Commons underground car park.
In 1984, they blew up the Grand Hotel in Brighton and nearly wiped out Margaret Thatcher's entire Cabinet. In 1991, they tried to kill John Major's Cabinet by firing mortars at 10 Downing Street.
So yes, we were lucky on Wednesday that it was 'only' a man in a rented car with a couple of knives. But we also owe an immense debt to the police and security services who have learnt well from the mistakes of the past. We are all immeasurably safer today than we were during the IRA bombing campaigns of the 1970s and 80s.
It would be the height of folly to claim that a coordinated series of attacks on the scale of the 7/7 bombings in 2005 could not be mounted again. But it is worth noting that for more than a decade, there has been nothing comparable. (In 2007, two car bombs were discovered and disabled before they could be detonated, and the following day, there was an attempted attack at Glasgow airport.)
Londoners like to claim that the spirit of Blitz lives on in the capital. The truth is that in all the major cities of Europe that have been attacked, life goes on. Which is, of course, exactly as it should be. Twenty-four hours after the Westminster attack, I walked through the heart of London's West End -- and with the exception of a helicopter whirring noisily overhead and a couple of heavily-armed police officers on patrol in Leicester Square, it was as if nothing at all had happened.
Over the coming days, we will learn more about the man who was responsible for the attack and perhaps begin to understand more about the best way to minimise the risk of more such attacks in the future. For the police and the security services, the task is never-ending -- to find, identify and monitor those who seek to do us harm. As the IRA said after they failed to kill Margaret Thatcher in 1984: 'Today we were unlucky, but remember we only have to be lucky once. You will have to be lucky always.'
The task for the rest of us is crystal clear: we keep calm and carry on. Because that's the exact opposite of what the killers want us to do.