Washington (CNN) Commerce Secretary Wilbur Ross was asked Sunday about whether or not President Donald Trump would go through with planned tariffs on steel and aluminum imports this week.

His response is incredibly telling -- about a lot more than just Trump's decision on tariffs. Here's the exchange between Ross and NBC "Meet the Press" host Chuck Todd (bold is mine):

TODD: So this is going to happen this week for sure? The way he said it, 25%, 10%?

ROSS: Whatever his final decision is is what will happen.

TODD: Meaning this isn't a done deal.

ROSS: I didn't say that. I just said what he has said, he has said. If he says something different, it'll be something different. I have no reason to think he's going to change.

Remember that Ross has known Trump for a very long time. He first met Trump in the 1990s when he represented a group of investors considering whether or not to remove Trump as the head of the Trump Taj Mahal casino in New Jersey.

He gets how Trump works. And thinks. And is.

Ross knows, then, what it has taken -- and continues to take -- those covering Trump years to figure out: He just says stuff. This is a day-to-day presidency. What Trump says on a Monday isn't indicative of what he will say on a Tuesday -- and certainly not telling of what he says on a Friday.

And, unlike most politicians, Trump simply doesn't feel any compunction about saying one thing one day and something totally contradictory the next day -- or even the next hour. When he says it, he means it -- and he believes it. And when he says something else that is the opposite, he believes that too.

There are examples of this tendency everywhere.

The open meeting on immigration with senators and House members in which Trump suggested that he was open to comprehensive immigration reform followed by Trump's adoption (or re-adoption) of hardline principles on the issue two days later.

Or Trump's pledge last Thursday -- again in a meeting with lawmakers -- that he wanted a comprehensive gun bill followed by his meeting, later that night, with the NRA. (It was his second meeting with top executives from the NRA in the week.) And the fact that we still have no clear legislative path forward on guns in the House or Senate.

Trump is hugely mercurial. He is easy to anger. He acts unilaterally. And when you do that, what you say in one moment can change.

Which, according to NBC News reporting , matters when it comes to Trump's decision on tariffs. This paragraph is a wow:

"Trump unilaterally made the decision to announce the new taxes on foreign imports in a fit of rage about a number of other issues that were weighing on his administration. Citing an internal document, NBC reported that White House lawyers and staff had not conducted any review of the drastic change Trump suddenly made to US trade policy."

That reporting on Trump's mood over the last week is consistent with lots of other stuff out there, including this from The Washington Post

"In an unorthodox presidency in which emotion, impulse and ego often drive events, Trump's ominous moods manifested themselves last week in his zigzagging positions on gun control; his shock trade war that jolted markets and was opposed by Republican leaders and many in his own administration; and his roiling feud of playground insults with Attorney General Jeff Sessions."

Trump rages and makes decisions on a whim. He later often reconsiders those decisions. He sees no issue in dong so.

And, when he was simply a businessman in charge of his own company, there wasn't. It's a little odd as a management approach but there are plenty of people who take all sorts of unorthodox approaches to business.

The problem now is that Trump is the President of the United States. His words -- much less his actions -- have sweeping consequences. What he says when he is "in a mood" is treated just like what he says when he is totally calm and even-keeled. World markets -- and the world -- see no difference since they can't see inside the White House to judge for themselves what sort of mood the President was in at the moment he said something.

Trump's willingness to just say stuff -- often ill-considered and driven by frustration, anger or resentment -- is perhaps the strongest marker of how different he is from every person who had held the White House prior to him. And why covering him poses a unique challenge for journalism.

What does Trump really think about any issue, person or country? It, literally, depends on the day.