Discussing his new book, Understanding Trump, with Breitbart editor-in-chief Alex Marlow on Monday, former Speaker of the House Newt Gingrich explained why he believes Donald Trump is one of the smartest people ever to have served as President of the United States.

“What always surprises members of the elite is I believe he is one of three or four smartest people ever to be president,” Gingrich said, adding:

I think he’s in the same league as someone like Lincoln or Theodore Roosevelt … I also think he’s one of the most energetic since Roosevelt, which is why you get these tweets at five in the morning, when he wakes up he has too much energy. I think also he has an instinct for disruption comparable to Andrew Jackson… He is constantly looking for ways to fix things, to change things, to breakthrough.

Having served as an informal advisor to Trump throughout his presidential campaign, Gingrich’s new book, Understanding Trump, seeks to establish what propelled the former businessman to the most powerful job in the world, as well as presenting a blueprint for how he can successfully implement his agenda.

During the interview, Gingrich also described “the two sides of Trump” — one being the “very pugnacious, risk-taking, live in the moment kind of guy, very existential. He’s the one who tweets.”

“Then there’s this very serious, long-term, thoughtful, disciplined guy. I don’t know if he’s a Gemini, I think he is a Gemini. There’s a sense of twins who occupy this guy, and I think there’s a lot of stories in here that people will find very interesting.”

Newt Gingrich’s new book, Understanding Trump, debuted at #1 on NYT best seller list and remains on the list for the fifth consecutive week. It is currently available to buy on Amazon and in all major book stores.