It may rank as the most notorious single claim in the history of philosophy. ''Until philosophers rule as kings in their cities,'' Socrates casually tells his young friend Glaucon, ''or those who are nowadays called kings and leading men become genuine and adequate philosophers … cities will have no rest from evils.''
This startling assertion comes some distance into Plato's dialogue Republic - at 473d, in the conventional pagination - but it introduces the work's main character, the so-called philosopher-king.
Socrates has defined the philosopher as not just a lover of wisdom but as a special kind of seer, someone dedicated to knowledge of capital-T truth. It follows that this exceptional fellow is the sole person fit to rule any city, including the ideal city he is sketching for his interlocutors.
We might immediately wonder: does he, or Plato, mean this seriously? There is a good deal of destabilising evidence. Socrates himself says a couple of times that he hesitates to make the claim, knowing how odd it will sound. And in the part of the quotation I elided above, he notes that existing philosophers, assuming there are any, will probably have to be forced to rule.
Elsewhere in the dialogue there are scattered clues that the whole ideal-city set-up, including the philosophically minded ruler, is a veiled warning that thinkers ought to steer well clear of politics. Force and deception will be necessary to turn an unruly populace towards the truth, he notes, without mentioning that this seems to set up a performative contraction: how can a loyal servant of the truth use deception as means even to a good end? And, in a blood-chilling passage, Socrates drops a hint that no ideal city will be possible without first getting rid of everyone over the age of 10. Call it the Clean-Slate Premise. Ouch.
Despite all this, Plato will be forever associated with the idea of the philosopher-king, and indeed the notion of a perfectly enlightened ruler is a spectre that haunts all politics. Every elected official, from the lowliest alderman to the president of a major nation, is doomed to be measured against, and fall short of, this towering ideal of perfect knowledge in the service of justice.
At the same time, the idea of a philosopher-king sounds a different kind of warning: not for philosophers to avoid politics, but for citizens to be on guard when any self-styled thinker or social engineer gets his hands on the reins of power. ''Quis custodiet ipsos custodes?'' the Roman poet Juvenal wondered, in his Satires - ''Who guards the guardians?'' (Or, if you are an Alan Moore fan, ''Who watches the watchmen?'')
It is a very good question, especially when those guardians come armed with some big-plan ideology, a few willing henchmen, and a taste for utopian social reform. Commitment to the truth sounds like a good thing, but experience shows that implementing an ideal social scheme all too quickly gets all too messy.
Plato himself was wary of political power. The treatment of his philosophical master, Socrates, under both oligarchy and democracy, was not encouraging; it was the latter form of rule that led to the frame-up trial which sentenced Socrates to execution by hemlock, which goes some distance to explaining the strong anti-democratic flavour of Plato's thought. His own attempt to mould Dionysius the Younger of Syracuse into a sort of philosopher-king was an abject failure. The youthful tyrant was addicted to luxury and the indulgence whim, and found his Greek visitor's epistemological advice tiresome.
Invoking this story, the critic Mark Lilla has thus spoken of ''the lure of Syracuse'': an irresistible temptation among certain intellectuals to set the political world to rights, usually with disastrous results. Nobody has so far asked me, but I figure I have the answer to the problem of the philosopher-king. Don't worry, it's not to grant me absolute power, much as I might covet that on certain days. It is, instead, to borrow a page from a different, and more ironic, tradition of ancient wisdom than the Greek philosophers.
Jacques Derrida, puzzling over the problem of the modern university, suggested that the best course to follow was to have a philosopher in charge of each and every one of them. Some of my colleagues seem bent on making this happen: philosophers are over-represented in university administration. But Derrida went on to note that no actual colleague, however brilliant, is sufficiently enlightened to qualify as a true philosopher. Therefore the chair of the university president should remain empty.
The empty chair is a striking part of the ethic of hospitality enacted by the Seder dinner: a chair for the guest who may arrive at any moment, for whom a place must be kept.
The practice has analogues in other places. Gatherings of PEN, the international freedom of expression group, always feature an empty chair for a missing writer, in prison or under house arrest elsewhere in the world. Somewhat less sublimely, Amazon chief executive Jeff Bezos apparently insists on having an empty chair at every company meeting: the chair represents the customer, according to Bezos ''the most important person in the room''. There it is - not the customer, I mean, but the chair. Instead of staging acrimonious elections for the post of president or prime minister, rather than arguing over who mismanaged a budget or failed to lower unemployment rates, we should simply hold regular viewings of the empty chair at the summit of all governance.
Behold the absent philosopher-king, the unfeasible ideal ruler, whose always imminent, always postponed arrival may guide us in the endless self-and-other relation that is politics.
See - how infinitely, impossibly wise!
Mark Kingwell is professor of philosophy at the University of Toronto.