Stephen Hawking, the great explainer of cosmic mysteries, is also known for sharing his wisdom on more earthly matters. He has weighed in on climate change (bold action is needed), boycotting Israel (he favored it), and the departure of Zayn Malik from the boy band One Direction (he could still be in the band in a parallel universe).

But on ITV’s “Good Morning Britain” on Tuesday, Mr. Hawking, a theoretical physicist and best-selling author, acknowledged a mystery that he has found impenetrable.

Asked to explain the political ascendance of Donald J. Trump, he said simply, “I can’t.”

“He is a demagogue who seems to appeal to the lowest common denominator,” he said in prerecorded remarks.

A message left with the Trump campaign on Tuesday was not immediately returned. The subject did not come up at a news conference that the Republican presidential candidate held in Manhattan on Tuesday.