As every schoolchild knows, the gravitational pull of the sun helps hold the planets in their orbits. Gravity from the centre lends coherence to the whole solar system.
I mention this because that's how our political and social systems used to work, but no longer do. In each sphere of life, there used to be a few big suns radiating conviction and meaning. The other bodies in orbit were defined by their resistance or attraction to that pull.
But now, many of the big suns in our world today lack conviction, while the distant factions at the margins of society are full of passionate intensity. Now, the gravitational pull is coming from the edges, in sphere after sphere.
In the 1990s, the central political institutions radiated confidence, derived from an assumed vision of the post-Cold War world. History would be a slow march towards democratic capitalism. Nations would be bound in peaceful associations like the European Union. The United States would oversee a basic international order.
This vision was materialistic and individualistic. Nations should pursue economic growth and a decent distribution of wealth. If you give individuals access to education and opportunity, they will pursue affluence and personal happiness. They will grow more temperate and "reasonable".
Since 2000, this vision of the post-Cold War world has received blow after blow. Some of these blows were self-inflicted. Democracy, especially in the US, has grown dysfunctional. Mass stupidity and greed led to a financial collapse and deprived capitalism of its moral swagger.
But the deeper problem was spiritual. Many people around the world rejected democratic capitalism's vision of a secular life built around materialism and individual happiness. They sought more intense forms of meaning. Some of them sought meaning in the fanaticisms of sect, tribe, nation or some stronger and more brutal ideology. In case after case, "reasonableness" has been trampled by behaviour and creed that are stronger, darker and less temperate.
A group of well-educated men blew up the World Trade Centre. Fanatics flock to the Middle East to behead strangers and apostates. China's growing affluence hasn't led to sweetening, but, in many areas, to nationalistic belligerence. Iran is still committed to its radical eschatology. Russia is led by a cold-eyed thug with a semi- theological vision of his nation's destiny. He seeks every chance to undermine the world order.
The establishments of the West have not responded to these challenges by doubling down on their vision, by countering fanaticism with gusto. On the contrary, they have lost faith in their own capacities of understanding and action.
Sensing a loss of confidence in the centre, strong-willed people on the edges step forward to take control. This happens in loud ways in the domestic sphere. The uncertain Republican establishment cannot govern its own marginal members, while those on the edge burn with conviction. Mr Jeb Bush looks wan but Mr Donald Trump radiates confidence.
The Democratic establishment no longer determines party positions; it is pulled along by formerly marginal players like Mr Bernie Sanders.
But the big loss of central confidence is in global governance. The US is no longer willing to occupy the commanding heights and oversee global order. In region after region, those who are weak in strength but strong in conviction are able to have their way. Russian President Vladimir Putin in Crimea, Ukraine and the Middle East. President Bashar al-Assad crosses red lines in Syria. The Islamic State in Iraq and Syria spreads in Syria and Iraq. Iranian proxy armies roam the region.
Republicans blame US President Barack Obama for hesitant and halting policies, but it is not clear that the foreign policy and defence apparatus believes any more in its own abilities to establish order, or that the American public has any confidence in US effectiveness as a global actor.
Some leader has to be able to digest the lessons of the last 15 years and offer a revised charismatic and persuasive sense of America's historic mission. This mission, both nationalist and universal, would be less individualistic than the gospel of the 1990s, and more realistic about depravity and the way barbarism can spread. It would offer a goal more profound than material comfort.
NEW YORK TIMES