In head

For now, it seems the the notion that the U.S. is about to go to war with North Korea is mostly in Trump’s head, but North Korea’s increasing nuclear capabilities are real, as are the thousands of artillery cannons that could rain down 300,000 rounds on South Korea in the first hour of war. Trump’s statements may only notionally represent foreign and military policy, but the safety of people from Seoul to Guam depends on the government of North Korea understanding that .

What does 'in Trump's head' mean?