Former Secretary of State Rex Tillerson, whom President Trump fired with a tweet nearly nine months ago, spoke about his experience with the president for the first time on Thursday evening in an interview with CBS News.

In the interview, which occurred at a fundraiser in Houston, Tillerson described an “undisciplined” president with whom he shared no “common value system” and talked of his frustration over Trump’s tendency to demand action that would illegally go beyond his executive powers.

In fact, Tillerson indicated that he believes he was ultimately fired over his tendency to push back on the president’s unreasonable or illegal demands. “When the president would say, ‘Here’s what I want to do and here’s how I want to do it,’ and I would have to say to him, ‘Mr. President, I understand what you want to do, but you can’t do it that way. It violates the law,’” Tillerson told CBS reporter Bob Schieffer. “I think he grew tired of me being the guy every day that told him, ‘You can’t do that, and let’s talk about what we can do.’ ”

It’s likely not the first time Tillerson has spoken critically of Trump. In October 2017, NBC News reported that Tillerson muttered to his colleagues that Trump was a “fucking moron” after the president made a sudden and arbitrary demand to build thousands of unnecessary nuclear weapons. (His spokeswoman said he never called Trump that.)

But Tillerson was a little more cautious in his public remarks Thursday. “He acts on his instincts, and in some respects it looks like impulsiveness. … But [it is not his] intent to act on impulse. I think he really is trying to act on his instincts,” he said.

But his depiction of the president’s unorthodox style did not come off positively: “It was challenging for me coming from the disciplined, highly process-oriented ExxonMobil Corporation to go to work for a man who is pretty undisciplined, doesn’t—doesn’t like to read, doesn’t read briefing reports, doesn’t—doesn’t like to get into the details of a lot of things but rather just kind of says, ‘Look, this is what I believe and you can try to convince me otherwise, but most of the time you’re not going to do that.’ ”