PARIS — Changes, disruptions, earthquakes ... We no longer know what word best describes the political events unfolding before our eyes. After former French president Nicolas Sarkozy lost in the conservative primaries, and current Socialist president François Hollande chose not to run, Matteo Renzi resigned as Italian prime minister, becoming the latest European leader to walk out the door. Add to that Britain’s exit from the European Union, Donald Trump's victory in the U.S., and upcoming national elections in Germany, and you'll have an idea of just how much old landscapes are breaking apart.

These mutations reveal the volatility of voters, their exasperation and their lassitude. They show the growing influence of mood changes on political life to the detriment of rationality and long-term consequences. As a result, some fear that democracy is deteriorating under the pressure of populism. There's a word in ancient Greek to describe this phenomenon: ochlocracy. It's no longer used. And yet, it deserves to be brought back in fashion.

"Ochlos" refers to the mob in its chaotic, tumultuous, messy, unpredictable character, as opposed to "demos" the people. Democracy is the power of the people but it takes place within a legal framework people created for themselves. Decisions come from citizens who use logic to deliberate — they have to be educated and knowledgeable to have informed and reasonable opinions. On the other hand, when the crowd dominates, when public emotions reign, the system subsides.

Floating at the whim of popular fervency, carried away by the masses, democracy risks sinking from pressures. If that happens, if the crowd replaces the people, a different regime takes over. That regime is ochlocracy. This is what the ancient Greek historian Polybius theorized in book six of Histories, in which he attempted to sum up classic Greek political thought.

Few read Polybius nowadays. Yet he was, in his time, which was between 200 and 120 BC, a leading figure of Greek history. A general after Alexander's death, he was taken hostage by the Romans, before the statesman in him turned into a historian, diplomat and political theorist. Few texts have had as much importance over the centuries. His works profoundly influenced Cicero, but also Machiavelli and Rousseau, to name just a few important historical figures.

What we've mostly retained from Polybius, as the French philosopher Jean-Claude Milner reminds us in his most recent book Relire la révolution (“Rereading the revolution”), is his theory of a cyclical evolution of political regimes, according to which different political systems succeed one another, from monarchy to ochlocracy, as if the wheel of history spins indefinitely.

Let's leave aside this cyclical notion. Let's leave aside even the idea that the power of the mob corresponds to a distinct political regime. There's never been a clear and convincing example of what a real ochlocracy could be. But that by itself is not enough to rid ourselves of the question. It’s this ancient idea that refers to the tensions that oppose, inside any democracy, the power by the people and the people itself. Demonstrations against decisions adopted in all democratic legality show that only too well.

There's an interminable debate about this question: Which is more legitimate? Is it the expression of anger, the crowds that demonstrate? Is it the representatives in parliaments and governments, the legislator and the executive? In other words, is democracy based on laws or might?

Without ever being entirely solved, this question is now transformed by social media, through the medium’s immediacy and its viral nature. When we consider what's in store for Europe and other parts of the world in the foreseeable future, these debates will certainly continue. What distinguishes democracy and ochlocracy isn't a matter of academia. It's a question about the future.