Mr. Trump doesn’t know the difference between the Quds Force and the Kurds. He can’t identify the nuclear triad, the American strategic nuclear arsenal’s delivery system. He had never heard of Brexit until a few weeks before the vote. He thinks the Constitution has 12 Articles rather than seven. He uses the vocabulary of a fifth grader. Most damning of all, he traffics in off-the-wall conspiracy theories by insinuating that President Obama was born in Kenya and that Ted Cruz’s father was involved in the Kennedy assassination. It is hardly surprising to read Tony Schwartz, the ghostwriter for Mr. Trump’s best seller “The Art of the Deal,” say, “I seriously doubt that Trump has ever read a book straight through in his adult life.”

Mr. Trump even appears proud of his lack of learning. He told The Washington Post that he reached decisions “with very little knowledge,” but on the strength of his “common sense” and his “business ability.” Reading long documents is a waste of time because of his rapid ability to get to the gist of an issue, he said: “I’m a very efficient guy.” What little Mr. Trump does know seems to come from television: Asked where he got military advice, he replied, “I watch the shows.”

Mr. Trump promotes a nativist, isolationist, anti-trade agenda that is supported by few if any serious scholars. He called for tariff increases that experts warn will cost millions of jobs and plunge the country into a recession. He claimed that Mexican immigrants were “bringing crime” even though research consistently shows that immigrants have a lower crime rate than the native-born. He promised that Mexico would pay for a border wall, even though no regional expert thinks that will ever happen.

Mr. Trump also proposed barring Muslims from entering the country despite terrorism researchers, myself included, warning that his plan would likely backfire, feeding the Islamic State’s narrative that the war on terrorism is really a war on Islam. He has since revised that proposal and would now bar visitors from countries that have a “proven history of terrorism” — overlooking that pretty much every country, including every major American ally, has a history of terrorism.

Recently, he declared that he would not necessarily come to the aid of the Baltic republics if they were attacked by Russia, apparently not knowing or caring that Article 5 of the 1949 North Atlantic Treaty obliges the United States to defend any NATO member under attack. Last week, Mr. Trump even invited Russia’s intelligence agencies to hack the emails of a former secretary of state — something impossible to imagine any previous presidential nominee doing. It is genuinely terrifying that someone who advances such offensive and ridiculous proposals could win the nomination of a party once led by Teddy Roosevelt, who wrote more books than Mr. Trump has probably read. It’s one thing to appeal to voters by pretending to be an average guy. It’s another to be an average guy who doesn’t know the first thing about governing or public policy.

The Trump acolytes claim it doesn’t matter; he can hire experts to advise him. But experts always disagree with one another and it is the president alone who must make the most difficult decisions in the world. That’s not something he can do since he lacks the most basic grounding in the issues and is prey to fundamental misconceptions.