As Emmanuel Macron savours his victory in the French presidential election, he might consider the words of John Maynard Keynes in an open letter, written to Franklin Roosevelt in December 1933. The British economist told the US president: "You have made yourself the Trustee for those in every country who seek to mend the evils of our condition by reasoned experiment within the framework of the existing social system. If you fail, rational change will be gravely prejudiced throughout the world, leaving orthodoxy and revolution to fight it out."
France does not have a global role comparable to the United States'. But it is certainly true that the success or failure of the new president will matter well beyond France - and even well beyond Europe. If Mr Macron succeeds, the forces of nationalism and political extremism - represented in France by his defeated opponent, Marine Le Pen - will suffer a setback worldwide. But if he fails, populism, nationalism and protectionism will soon be resurgent.
For while Mr Macron can savour a crushing victory over Ms Le Pen, he also knows that 35 per cent of French voters have just voted for a far-right candidate. The cumulative vote for extremists of the far left and the far right in the first round of the presidential election was closer to 50 per cent. That means that almost half of French voters want to smash "the system".
It is Mr Macron's job to show that the system can work better. If he fails, then, as Keynes put it in the 1930s, "rational change will be gravely prejudiced throughout the world".
The chances of failure are quite high. Mr Macron has to reinvigorate the French economy as well as the "European project". Both are notoriously difficult to reform and face deep structural challenges that might defeat even the most imaginative and dynamic politician.
The tasks of reform at home and in Europe are linked. Unless he can demonstrate to the German government that France is genuinely changing, then the Germans are unlikely to take the risk on the much-deeper integration of the European Union that Mr Macron thinks (probably correctly) is necessary to make the European single currency work.
At home, the challenges he faces are pretty obvious. The French state is abnormally large, with government spending accounting for 56 per cent of gross domestic product. The private sector is over-regulated and the public finances are over-stretched. Reducing the size of the state and making the labour market more flexible should help to generate jobs and economic growth. But any efforts at neo-liberal reforms will inevitably face passionate resistance from the far left, the far right, the unions and a large part of the political establishment. Street demonstrations have stopped previous efforts at economic reform in their tracks for 20 years and more.
Mr Macron's domestic political base is also fragile. He is, in some ways, an accidental president whose victory was achieved partly because the traditional centre-right and centre-left parties chose unelectable candidates, hamstrung either by extremism or by personal scandal. There is a strong chance that En Marche!, Mr Macron's new political movement, may not gain enough seats in parliamentary elections next month to allow the new president to get his agenda through, without forming an unstable coalition.
But Mr Macron's position as a political newcomer could also be an advantage if he can carve out a new space in the political centre ground. As a former minister in a Socialist government, he could send a bold signal by appointing a prime minister from the opposite political tribe, the centre right. If he can pull in enough support from the right, while retaining the support of the reformist wing of the Socialist party, he could yet create the backing he needs to push through reforms - for example to the country's 35-hour working week.
The problem of opposition on the streets will be real and will have to be faced down. An early pay increase for the police, many of whom will have voted for Ms Le Pen, might be advisable.
If Mr Macron can demonstrate that he is really reforming France, he might then gain the credibility to go to Berlin and demand reforms to the EU. The government of Chancellor Angela Merkel has been understandably wary of French calls for a loosening of austerity in Europe or for the issuing of common EU debt, believing that they ultimately come down to a desire that thrifty German taxpayers should fund the profligate French state. But there is also a growing awareness in some parts of the German government (the foreign ministry more than the finance ministry) that a failure to give some ground to France and Italy could ultimately prove disastrous for Germany itself, if it means that reformists like Mr Macron fail, and are replaced by radical populists such as Ms Le Pen.
The election of Mr Macron will also be greeted warily in London, where it is feared that his passionate defence of the EU will translate into a particularly tough line on Brexit. But a more confident France and a revived EU might be less inclined to see Brexit as a mortal threat, and so more willing to strike a win-win deal that keeps markets open and alliances intact.
There really is a lot riding on the success of President Macron - and not just in France.