It turns out that Heraclitus, the Greek philosopher, knew exactly what he was talking about some 25 centuries ago when he wrote: ”Character is destiny.” Consider American presidents who have disappointed, even failed, in office – theirs were not failures of intellect or education. Most almost certainly aced their SAT tests. Almost without exception, they were instead failures of character.

American voters intuitively grasp that truth. That’s why we look at our presidential nominees’ records in public office, to see if they have demonstrated – in the causes they dared to lead or in the roll-call votes they had to cast – courage, constancy, vision and character.

Americans have also prized courage and character in wartime in selecting their national leaders – consider Generals George Washington, Andrew Jackson, Ulysses Grant and Dwight Eisenhower, Colonel Teddy Roosevelt, and Lieutenant John Kennedy.

But for the first time in history, one of the nominees, real estate billionaire Donald Trump, has no record of public service to consider. Trump has never held any elected or appointed office. He has never served in the nation’s military. He didn’t join the Peace Corps. So while Donald Trump is more than eager to tell voters what he has made, how are voters, more importantly, supposed to find out what Donald Trump is made of?

Donald Trump’s self-proclaimed credentials to be president are his record in business and his financial success, in addition to, as he regularly reminds listeners, his own admirable generosity in charitable giving, especially to U.S. military veterans’ causes. How can we find out if Trump really is as successful as rich, as generous, and as civic-minded as he insists he is – whether Trump really is a man of character?

Every president since Harry Truman (who paid 38 percent of his $74,000 income in taxes), with the exception of the unelected Jerry Ford, has concluded he owed it to American voters to publicly disclose his personal tax returns.

As recently as 2012, Republican Mitt Romney, whom Donald Trump endorsed for president that primary season, was criticized by Trump on Greta Van Susteren’s Fox News show, for being too slow in making public his tax returns.

When Romney eventually did release his returns in September 2012, voters learned that, because of the lower tax rates on capital gains and dividends, Romney, on his nearly $14 million of income, paid just 14 percent in federal taxes.

But we also learned that, personally, Mr. and Mrs. Romney in 2011 had made generous charitable contributions totaling $4 million.

Donald Trump, having previously promised to make public his tax returns, now refuses to do so. Trump in order to qualify for casino licenses in New Jersey and Pennsylvania did disclose his income taxes as those states required. It was revealed that in 1979 and 1980, Trump (who had earlier boasted that he was already worth $200 million) had paid not a dime in U.S. income taxes. Interesting values: To get a license to operate blackjack tables or slot machines, Trump was willing to disclose his tax returns, but not to secure the trust of the nation to become president?

Whether this boastful billionaire continued to pay taxes at a lower rate than a firefighter or a night-shift nurse paid, and whether he benefits from Swiss bank accounts or Cayman Island investments, and how personally generous he is in his charitable donations will tell us volumes about the Republican standard-bearer’s true character. And when it comes to presidents, as we have painfully learned, character is indeed destiny.

Editor’s note: To find out more about Mark Shields and read his past columns, visit Creators Syndicate at www.creators.com.