Legendary investor Warren Buffett gave a simple argument for why he’s pro-immigration.

“Well, immigration, this country is built on it. I always say to people that are anti-immigration, ‘let’s put it in retroactively.’ And everybody leaves,” the 86-year-old billionaire said on a panel with his friend Bill Gates on Friday at Columbia University in New York.

He continued: “This country, if you think about it, Bill’s heard this before from me — we are sitting here, in part, because of two Jewish immigrants who in 1939 in August signed the most important letter perhaps in the history of the United States.”

He recommended that the audience search the letter online.

That letter was sent to President Franklin D. Roosevelt by Albert Einstein and Leo Szilard warning that Germany might develop an atomic bomb. It resulted in the Manhattan Project.

“If it hadn’t been for those two immigrants, who knows whether we’d be sitting in this room,” Buffett said.

He later added: “This country has been blessed by immigrants. And you can take them from any country you want and they’ve come here and they’ve found something that’s unleashed the potential that the place where they left did not, and we’re the product of it.”

On Friday afternoon, President Donald J. Trump was unveiling a pair of executive orders, including one reportedly calling for a temporary halt to all refugees.

“Secondly, I’m establishing new vetting measures to keep radical Islamic terrorists out of the United States of America. We don’t want them here. We want to ensure that we are not admitting into our country the very threats our soldiers are fighting overseas. We only want to admit those into our country who will support our country and love deeply our people. We will never forget the lessons of 9-11, nor the heroes who have lost their lives at the Pentagon. They were the best of us. We will honor them not only with our words, but with our actions. And that’s what we’re doing today. I am privileged to be here with you, and I promise that our administration will always have your back. We will always be with you,” Trump said according to a White House press pool report.

Earlier in the week, Trump ordered additional orders aimed to crack down on illegal immigration, including a measure expanding the authority of local law enforcement officers to enforce immigration laws, among other policies. Critics say the policy, known as “Secure Communities,” will make victims and other people less likely to cooperate with the police if they are worried about their immigration status. Trump also announced that it was his administration’s policy to immediately begin construction of a wall along the U.S.-Mexican border.