Special counsel Robert S. Mueller III after a meeting on Capitol Hill on June 21, 2017. (J. Scott Applewhite/AP)

As Robert S. Mueller III’s investigation has gotten closer to President Trump — most notably in Michael Cohen’s recent sentencing for felonies that implicated Trump as “Individual 1″ — observers are again asking whether the president of the United States can be indicted while in office. The debate stems in part from a 1973 Justice Department memo filed by then-Solicitor General Robert Bork, recommending that the president be exempt from criminal indictment while in office.

But the real issue is the rule of law, something mature democracies often take for granted. Political philosophers and theorists have long affirmed that functioning republics depend fundamentally on the rule of law. With that at issue, we may wish to reconsider the lessons of the West’s first democracy: the ancient Greek city-state of Athens.

Thucydides emphasized the rule of law

Unfortunately, Trump has lost Cabinet members such as H.R. McMaster and Jim Mattis, among the administration’s keenest readers of Greek history. If they had continued to enjoy the president’s confidence, they might have reminded him of their favorite historian, Thucydides, who chronicled the Peloponnesian War. As Thucydides documents, Athens’s most celebrated democratic leader was Pericles, who was charged with keeping the city’s spirits up amid a violent and relentless war against Sparta. To do so, Pericles worked to inspire citizens by appealing to their shared democratic values — especially the rule of law, reportedly declaring, “We respect the law greatly and fear to violate it . . . especially those laws that were made to help people who have suffered an injustice.”

Although Thucydides lived long enough to record most of the events of the war, he did not survive its aftermath. Sparta imposed tyrannical rule by a group of oligarchs known as “the Thirty.” In just eight months, these despots killed 5 percent of the city’s population and exiled many more. The surviving Athenians finally revolted to restore democratic rule, an uprising in which countless more Athenians died.

Plato emphasized the rule of law

Perhaps the most prominent tyrant among the Thirty was Critias, an uncle to the philosopher Plato. After witnessing this regime, in which no laws controlled the rulers’ whims, Plato knew that for a republic to thrive, its rulers must be held accountable to the law. This is especially the case, he argued, because those with unchecked power can become “swollen with insolence and injustice.” Elaborating on the proper relationship between rulers and the law, he wrote:

If I call those who are usually known as rulers ‘servants’ of the law, this is . . . because I think it is on this, more than anything else, that the safety . . . of the city depends. In this kind of city where the law is subordinate, and lacks authority, I see disaster just around the corner. Where the law is a master over the rulers, and the rulers are slaves to the law, there I see salvation, and all good things the gods can grant to cities.

To Plato, what most fundamentally distinguishes a thriving republic from one on the cusp of despotism is the subordination of its rulers to the laws. So long as the rules are upheld and indeed enforced upon the rulers, the city enjoys “all good things”; otherwise, the opposite. Plato’s student Aristotle would affirm the same in his “Politics,” observing:

. . . he who bids man rule [over the law] adds an element of the beast; for desire is a wild beast, and passion perverts the minds of rulers, even when they are the best of men.

Even an Aesop fable emphasized the rule of law

This principle was so fundamental to the Greeks that, well before Plato or Aristotle, Aesop immortalized it in his fable “The Frogs Who Wished for a King.” A free city of frogs had grown bored with their lives in the pond and demanded a king, who would entertain them and “rule them in a way to make them know they were being ruled.” The god Jupiter, amused by their foolishness, instead sent them a log — which, as scholars know, was an ancient symbol of the rule of law, because laws were frequently then inscribed on blocks of wood. The frogs mocked the gift, and, instead of using it as the foundation of their city, they used it as a diving board for some frolicking. Soon becoming bored again, however, they once more demanded a king from Jupiter, who this time gave them exactly what they demanded, in the form of a crane, who proceeded to devour them. When the carnage was over, Jupiter asked: “Are you not yet content? You have what you asked for and so you have only yourselves to blame for your misfortunes.”

For Greeks such as Pericles, Plato, Aristotle and Aesop, the choice was obvious. One may either have a rule of laws or the arbitrary rule of a despot. In placing a ruler beyond accountability to the law, the public makes a fateful choice. This is as good a time as any to recall their lessons.

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David Lay Williams is a professor of political science at DePaul University and the author of “Rousseau’s Platonic Enlightenment” (Penn State University Press, 2007) and “Rousseau’s ‘Social Contract’: An Introduction” (Cambridge University Press, 2014). He is writing a book for Princeton University Press, “‘The Greatest of All Plagues’: Economic Inequality in Western Political Thought.”