The single most important line from Trump's Tuesday talk-a-thon

(CNN)In the course of President Donald Trump's 55-minute public meeting with the top Democratic and Republican leaders in Congress on Tuesday, he said lots -- and lots -- of things. ("I hope we've given you enough material," Trump told reporters at one point. "That should cover you for about two weeks.")

But, one line amid all of those sentences stood out to me. It's this one -- on Trump's own beliefs when it comes to immigration policy:
"I think my positions are going to be what the people in this room come up with."
Consider that for a moment.
    No issue more defined Trump's candidacy than immigration. Very early on in the 2016 race, he identified the anger and frustration within the Republican base about the government's policies toward undocumented immigrants. His pledge to build a "big" and "beautiful" wall along the country's southern border -- and to make Mexico pay for it! -- was the foundation on which his campaign (and its appeal) was built. It was the cornerstone.
    And yet, in a meeting with all of the top congressional leaders in both parties, Trump made very clear that he was pretty much good with whatever they came up with on immigration. His position on how to balance border security, funding for the wall (more of a fence, Trump clarified Tuesday, and broken up with natural barriers like rivers) and the extension of the Deferred Action for Childhood Arrivals (DACA) program would be entirely dictated by what sort of deal they cut.
    "I'm signing it," Trump pledged at one point in the meeting about a -- really, any -- possible legislative deal. "I mean, I will be signing it." At another, he assured lawmakers "if they come to me with things I'm not in love with, I'm going to do it."
    The Trump on display on Tuesday seems irreconcilable with the Trump from the 2016 campaign or most of his first year in the White House. He portrayed himself in the campaign as someone with a rigid -- and totally un-PC -- belief system, willing to speak hard truths and stick by them even amid backlash. And yet, on Tuesday, he cast himself as a sort of un-partisan deal-maker -- more interested in getting to "yes" than standing on some sort of principle.
    Which side represents the real Trump? The cop-out answer is "both" -- and I do think there's some truth to that. Trump has proven himself throughout his life to be hugely mercurial and unpredictable. What he says today -- or even how he acts -- isn't predictive of what he will say or do tomorrow. He believes there is considerable value in keeping people on their toes, never truly knowing what he is thinking.
    That said, I do think that the long arc of Trump's life suggests that he, in his perfect world, would be a sort of unideological deal-maker. He fashions himself -- and always has -- as someone uniquely able to make deals.
    "I never get too attached to one deal or one approach," Trump wrote in "The Art of the Deal." "For starters, I keep a lot of balls in the air, because most deals fall out, no matter how promising they seem at first."
    My strong suspicion is that Trump views policy-making in a very similar light. The details of a policy aren't a huge deal to him -- hence his lack of specific policy knowledge. One example: On Tuesday, Trump seemed to agree with California Sen. Dianne Feinstein that a "clean" DACA bill be passed, meaning without any border security funding attached. House Majority Leader Kevin McCarthy, a California Republican, had to quickly step in to clarify before Trump blundered into a deal that most Republicans in Congress object to.
    To be clear: All politicians have fungible beliefs on most issues -- particularly those that get to the level where they can be considered credible candidates for president. Moving up the political ladder often necessitates a series of compromises.
    But, almost all politicians also have some policy area where they are unwilling to compromise -- an area untouched (or at least unchanged by) political concerns. The best recent(ish) example of that is John McCain's support for a surge of troops in Iraq in 2007, a move that was politically unpopular at the time but McCain believed was the right thing to do.
    What seems to distinguish Trump from most politicians is that there is no obvious issue on which he has a set of beliefs that he considers sacrosanct. There isn't any area where Trump thinks a deal or a compromise isn't possible. The deal is the thing, not the policies that make up the deal.
    That's something different than we've seen in the Oval Office before. But, when it comes to Trump, what else is new?