It is worth remembering that, despite everything, the world is a better place than it was. In terms of lives lengthened by medical science, clean water and more abundant food, improved by education and freedom, made more effective by democracy, more comfortable by prosperity and the reduction of cruelty and pain, the world is in a measurably better condition than when our parents or grandparents were born.

The Western ideals of humane, secular, liberal, carefully capitalist democracy have triumphed. Liberty, Equality and Fraternity are now to be found on every continent.

I remind myself, from time to time, that I was born into a Europe of fewer than a dozen democracies, and in the past 58 years the number has more than doubled. Dictatorships of the individual (Spain and Portugal), of the military (Greece) and of the people (Eastern Europe) have given way to a democratic much of a muchness, and all the better for it.

I am an optimist. It is right, of course, to rail against the setbacks, but  also sensible, occasionally, to celebrate our progress. Take this quiz and find out if you’re an optimist, too. Are you an optimist?

As for the dismal setbacks of the last few days, the answer to the threat posed by terrorism is not exclusion, but inclusion. The evidence suggests that human nature doesn’t differ radically from one place to another. Given half a chance,  most of us will choose safety, prosperity, freedom, even tolerance, rather than the empty promise of a utopia at the cost of brutality, starvation and personal danger. Most of us don’t want excitement.

Sadly, the idea of exclusion enjoyed a good day yesterday:

We can build a wall around the Middle East, as many might suggest, but let’s not forget to leave the oil pipes in place.

We can build a wall around Europe. But that would be both wrong (the vast majority of refugees are genuinely seeking safety, fleeing the very evils that we saw on the streets of Paris on Friday), ineffective (the terrorist will always get through), and withering (Europe needs new people and new ideas).

We can even exclude IS from ordinary consideration by calling them ‘psychopathic monsters’, as John Kerry did last night, in an otherwise moving speech, but more can surely be gained by trying to understand how people who must be essentially like us can come to be brutal and inhumane.

We can begin to exclude our own citizens (even more than we do already), as Marine Le Pen suggests. Send them home. Deny them citizenship. But narrow nationalism has a poor track record, even in Europe.

Societies are strengthened by inclusion. The Romans discovered this civilisation-building trick two thousand years ago, extending the rights of citizenship to the tribes they subjugated. The conquered flourished alongside their conquerors.

What should our Governments do?

Yes, spend more money on surveillance, and security, and if necessary ‘strip search’ new arrivals at our borders if that humiliation brings reassurance to the bigots, but let’s not destroy our own freedoms and become illiberal in the process.

Yes, ‘destroy’ IS if it’s possible, but ‘boots on the ground’, as President Obama calmly argued yesterday, haven’t won the wars in Afghanistan or Iraq, and they won’t win the war in Syria. Certainly, if we don’t minimise ‘collateral damage’ we won’t make friends of those who survive our interventions.

But, above all, let’s find a long-term way of bringing optimism, freedom and prosperity to the Arab world. And no, I don’t know how to do it.