According to Donald Trump’s own former secretary of state, the president has frequently proposed ideas that would “violate the law.”

While Rex Tillerson declined multiple opportunities to deny having called Trump “a fucking moron” while still serving in the president's Cabinet, he seemed to confirm the sentiment during a charity event Thursday night in Houston. Speaking with CBS News political contributor Bob Schieffer, Tillerson opened up about how working so closely with someone who often did not listen to experts, change his opinion based on new data, or consider international law when making plans was maddening. He admitted that he tried without success to persuade Trump to not bail on the Paris climate accords or the Iranian nuke deal.

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"I think part of it was obviously we are starkly different in our styles. We did not have a common value system," Tillerson told Schieffer. "When the president would say, 'Here's what I want to do and here's how I want to do it,' I'd 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. It violates a treaty.'"

Tillerson, who was fired back in March, did not say what actions Trump wanted to take that would have run afoul of the law.

"You know he got really frustrated," Tillerson said of Trump. “I didn’t know how to conduct my affairs with him any other way than in a very straightforward fashion, and I think he grew tired of me being the guy every day that told him, ‘You can’t do that.'"

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He said he told Trump he would be willing to “fight the fight” to change laws in Congress so the president could press forward with his agenda.

“That’s very, very interesting,” Schieffer said to laughter.

WATCH: Former Secretary of State Rex Tillerson is speaking publicly about what led to his firing in March by President Trump. CBS News political contributor @bobschieffer interviewed Tillerson in Houston last night, at a dinner to benefit MD Anderson Cancer Center. pic.twitter.com/47qDqcsrMs — CBS This Morning (@CBSThisMorning) December 7, 2018

Tillerson's revelation comes just as Trump's one-time FBI director returns to testify before Congress. The last time James Comey appeared on Capitol Hill, he testified under oath that Trump pressured him to drop an investigation into former national security adviser Michael Flynn — a move that could be interpreted as obstruction of justice.

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After noting that he had never met Trump until the day he was named secretary of state, Tillerson said “What was challenging for me coming from the disciplined, highly process-oriented Exxon Mobil Corporation was to go to work for a man who is pretty undisciplined, doesn’t like to read, doesn’t read briefing reports, doesn’t like to get into the details of a lot of things, but rather just kind of says, ‘This is what I believe.’ ”

It may be useful to recall why Tillerson reportedly called the president "a fucking moron" in the first place:

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Trump was in a meeting discussing the size of the U.S. nuclear arsenal compared to Russia. Upon seeing a graph showing the U.S. stock declining, he said that the nation should increase our nuclear weapons “by ten times!” When pressed by others as to whether he really meant by a factor of ten, he continued to insist that was what he actually meant. Tillerson soon stormed out of the room and threatened to resign, telling Vice President Mike Pence that Trump was “a fucking moron."

The president was quick to respond to Tillerson on Twitter on Friday:

Interestingly, Tillerson remarked on the president's Twitter habit on Thursday.

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“I will be honest with you: It troubles me that the American people seem to want to know so little about issues — that they are satisfied with 128 characters,” Tillerson said. Ultimately, of course, Tillerson was ousted from his Foggy Bottom post via Twitter.