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.

Here's how Brennan responded:

"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." [emphasis added]