If, after the Paris slaughters, there is to be a war, let it first be a war of ideas armed with the only weapons that have not yet been tried: the principles of social decency; the norms on which our daily lives rest, most often untested, unspoken, untried as we go about our daily business of making a living, raising a family, a bit of art here, a bit of sport there, some music, some cooking, some loving.

Until Friday night I thought “the pursuit of happiness” an anomaly among the “self-evident” truths of the American Declaration of Independence; Thomas Jefferson’s Hallmark card sentiment beside the bigger nostrums of “life” and “liberty”. But the “carefully chosen targets” of the Islamist militants of Isis were exactly the places where we go to pursue our little share of weekend pleasure: the football ground, the music hall, the café, streets such as those around the Canal St-Martin where people stroll and sit, joke, gossip, flirt. These are spots where friends find that bounce of delight in one another’s company; where strangers — often of different tongues, countries, habits of belief — ex­change glances and chat with something other than massacre on their minds.

Suddenly these places, characterised by the pleasure-exterminators of Isis as dens of “prostitution and vice”, seem bigger than mere resorts of casual entertainment. Instead, they seem to be at the heart of the urban innocence, emblems of the openness that the assassins of joy want to turn into a graveyard where sociability is patrolled by the morality police. Isis would see them turned into somewhere spontaneity is replaced by the tyranny instituted in its Syrian stronghold of Raqqa, a place of tuneless silence, fear, torture and rape. It would create a place where public entertainment and education is the summons to a public execution; a landscape with mass graves of the kind discovered after the Kurdish liberation of the Iraqi town of Sinjar. Among the most horrifying aspects of Friday’s outrage was the reported youth of the mass murderers: young men with the usual moronic shouts invoking godly legitimacy on their atrocity, visiting death on their own generation.

After 9/11 I believed refusing to be cowed into silence would be the best revenge. After this year’s Charlie Hebdo murder, I hoped the magazine’s satirists would not lose their taste for scandalous disrespect. It is to be hoped the kindness of strangers is not wiped out by terror.

And even in the midst of the bloody mayhem, there were potent signs of common decency: the French football team refusing to leave their German opponents when the latter were unable to return to their hotel, threatened as it had been by bomb threats; the #portesouvertes hashtag that sprang up on Twitter to open doors to anyone unable to make it back to a place of safety.

But the instincts of human sympathy and solidarity are not enough to get us through the onslaught. Christmas shopping is not a counter-attack. What our fellow citizens need now is a clarifying, empowering and inspiring statement of just what it is we must defend, if necessary, to the end. This, rather than the fluctuations of the business cycle, ought to be on the agenda of this week’s summit of the Group of 20 leading nations.

And what are those principles? They are the ones enshrined in the words of those who first articulated the imperatives of free speech: religious toleration; the right to civil peace; resistance to tyranny and theocracy. They are integral to the imperishable statements of Jefferson, John Milton and John Locke, but also Montesquieu, Voltaire, Condorcet, Emmanuel Levinas. They should be written on our battle standard, now that we know they cannot be taken for granted, as something to fight for.

First, the separation of religion from the power of the state.

Second, the right of people of different beliefs and none to share the same living space without oppression or intimidation.

Third, the right of utterance and publication so long as it precludes invitations to violence.

Fourth, the right of equality of the sexes, and of any sexual orientation, in all matters of law, education and employment.

Fifth, the right of all members of the same civil society, provided they have subscribed to these fundamental principles, to the vote, to education, to employment and to expression irrespective of ethnic background and faith.

These are all principles abhorrent to our enemy — the party of death. Against it we should be proclaiming these values as our new declaration of life, liberty and the pursuit of happiness.

The writer is an FT contributing editor

This article will remain open to comments until 6pm London time.

Letters in response to this article:

Ask Muslim leaders for their inspiring statement / From Corrado Pirzio-Biroli

Co-operation with Russia must be circumspect / From John Griffiths

To confront Isis, we must be united in our values / From John Morrison

Jefferson knew when action was called for / From David D Hebb