For years, John Brennan was a walking vault for top-secret government information.

Brennan was the director of the CIA until early 2017, but he says keeping those secrets was hardly the most important part of his job.

In an episode of Business Insider’s podcast, “Success! How I Did It,” Brennan spoke with Business Insider US editor-in-chief Alyson Shontell. Shontell asked Brennan how he dealt with the stress of knowing so much and not being able to share it with anyone.

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Here’s how Brennan responded:

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“No, I don’t know everything. Absolutely I don’t know. To me, it’s Socratic wisdom, which is you start to realize how much you don’t know and for an intelligence officer, even for others, it’s critically important to understand what it is that you know, the confidence that you attach to what you know.

“But most important for an intelligence officer is what you do not know, and briefing President Obama, President Bush, and others. I’d talk to him about what it is we know, reporting our confidence in our assessments and analysis, but I always made a point of saying that we don’t know this and we’re unsure of this, and we may have the opportunity to learn about this, but we’re not going to be able to find out about this before it happens.”

Brennan said he suspects the ability to admit when you don’t know something is more important today than ever before:

“One’s wisdom is understanding exactly how limited one’s knowledge is in this world, where there’s just been an explosion in information, which is maybe more challenging now because of social media and fake news and all the stuff that’s out there separating the wheat from the chaff.”

Admitting you don’t have all the answers can be challenging. Scientists use the term “illusion of knowledge” to describe what happens when “experts” are more likely to claim they know about fake concepts related to their area of expertise. In one study, self-perceived experts in biology said they knew fake terms like “meta-toxins” and “bio-sexual.”

In other words, admitting you don’t know something often means overcoming the human temptation to look smart.

But regardless of your job, it’s important to acknowledge the limits of your own knowledge – even and especially in meetings with your boss.

In a 2014 episode of the Freakonomics podcast, hosts Stephen Dubner and Stephen Levitt explained why “I don’t know” can be the best response when your boss asks you a question you can’t answer. Levitt said the alternative – feigning knowledge – is counterproductive.

That said, once you do admit your relative ignorance, Dubner said, you’ve got to “work like a dog to learn.”

In the interview with Business Insider, Brennan praised President George W. Bush for having the guts to admit when he didn’t know something.

“President Bush really was a very hard worker, very diligent,” Brennan said. “He’s somebody who recognized he didn’t have all the knowledge that he needed, but I felt that he was a wise person because he tried to tap into the knowledge of others.”