Brennan: There’s never been a previous president, at least in my lifetime and experience, who had the impulsivity that Mr. Trump exhibits. He frequently will tweet or say something or act upon his gut instincts, which apparently served him well in business. And it clearly served him well as far as the campaign. But he is woefully inexperienced in international affairs.

His approach is to tear down that which came before him and to criticize those who came before him without understanding the reason why certain policies were formulated and implemented. So his modus operandi is to do things in a very spontaneous fashion. And when you’re talking about international security issues, that could have tragic consequences, both in the near term as well as over the longer term.

In my experience with other presidents, they recognized the sacred obligations that they had as president. Before acting or speaking, they would internalize what the implications were and what the likely outcomes and reactions were going to be. Mr. Trump does not do that. I think he acts and speaks before thinking, a lot of times, about what the effects and impact are.

Friedman: [Are your reservations about Trump] primarily about differences with the specific policies he’s pursued? Are they more about the manner in which he’s pursued them? Or is it about his fitness to command more fundamentally?

Brennan: It’s a combination of concerns and factors. There are his traits, his personal qualities that are a concern. There is the lack of deliberate thinking that’s a concern. There is a lack of understanding and knowledge that’s a concern. And then there is this bullying tendency that compounds all of the other deficiencies.

Friedman: Are there certain actions that you think Trump would have taken had the “governors” [of his instincts] not been governing?

Brennan: He probably would have fully scuttled the [Iran nuclear deal] and not pushed it over to Congress, although I think both the governors as well as his political advisers saw that as a better choice because then he can blame others for it, which is his modus operandi. I think his initial, very negative statements about NATO would have continued to prevail if it wasn’t for individuals like a Jim Mattis and a Joe Dunford and others who explained to him exactly why Article 5 is so important. I think he probably would have embarked upon a foolhardy improvement in relations with Mr. Putin without any conditions or smart negotiation if it wasn’t for the hue and cry that has existed as well as congressional pressure.

Friedman: But sometimes a president’s impulses and instincts are what matters, ultimately, right? Aren’t there moments in a president’s job when advisers can only do so much?

Brennan: Absolutely. The judgment of any president is going to be crucial because a lot of times there will be recommendations and options, and there will be disagreements, and the president’s the one who has to decide. A lot of times there’s a time factor and he needs to make a decision quickly. This is where all those other features of Mr. Trump come into play because he doesn’t have the experience, he doesn’t have the expertise, and I don’t get the impression that he really tries to learn about these issues. Now I’m not in the Oval [Office]. But from everything I have heard him say and do, he hasn’t shown me that he’s a learning president.