Let me take a break from my ongoing nerdiness to return briefly to the now-au courant subject of “post-truth politics.” Andrew Sullivan has teed me up.

Sullivan has the cover story in the latest issue of Newsweek, called “The Democrats’ Reagan,” about the enormous potential of a second Obama term. One of his main arguments is that a decisive defeat for Republicans in 2012 could interrupt their rightward drift. Defeat would cause soul-searching, cooler heads would once again prevail, and the party would tack to the center. Color me skeptical.

Mostly I was struck by this line, which comes in the context of a discussion about immigration:

Under Obama, deportations of illegal aliens are double what they were under his predecessor; and the number of border agents is at a record high. Both give him conservative credibility on the issue, if only the right would acknowledge it.

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Read it again. See if you can spot the problem.

In post-truth politics, the basic mistake to see things like “credibility” as objective phenomena in the world. Put high heat to water, you get steam. Put conservative immigration policies to Obama, you get conservative credibility.

Credibility is not like that. It’s what you might call a relational phenomenon; it exists in the relationship between object and subject. Think about a rainbow. (Always good advice.) For a rainbow to exist, you need sunlight and water droplets in the air, but also a subject positioned at a particular angle to the sun and water droplets. A rainbow is just “how the light bouncing off the droplets appears to the subject.” Without the subject, there’s no appearing, and thus no rainbow.

Credibility is like a rainbow. It is relational. “Conservative credibility” is not something that simply happens when conservative policies are enacted or conservative rhetoric echoed. It requires a subject — in this case a conservative subject — to witness and acknowledge it. One must be credible to someone, and to have “conservative credibility” one must be credible to conservatives.

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Perhaps in the pre-post-truth era — the one for which Aaron Sorkin is so palpably nostalgic in The Newsroom — there were respected members of the elite who could effectively referee public life. If news anchors, major newspapers, or certain popular politicians deemed a perspective or person credible, it was taken seriously. (And the reverse: When Walter Cronkite deemed Nixon [oops, I mean LBJ] no longer credible on Vietnam, it was an important moment.)

But there are no referees any more, no members of the elite who transcend the partisan war and are respected by both sides. Or at least very few. There are only the sides and their respective worlds. Conservative credibility can only come from the conservative side, and if conservatives refuse to grant it, it doesn’t exist, any more than a rainbow exists when no one’s looking at it.

I’m not just making a rhetorical point. Sullivan’s formulation is indicative of a common mistake among centrists — most particularly, Obama himself! As I wrote back in one of my first post-truth posts, Obama came into office believing that if he made substantive concessions to conservatives, it would earn him credibility that he could then “spend” to gain bipartisan support for his own initiatives. That’s one reason he was always so gung-ho on oil and gas drilling. He assumed that conservatives would see the overtures on energy, grant him credibility, and come to the table on a climate solution.

What he missed — again and again, until the debt-ceiling fiasco — is that conservatives are not compelled by substantive policy concessions to grant him anything. There are no referees to compel them, nothing to stop them from calling him a job-killing socialist no matter what he does. And in this age of zero-sum politics, it’s in their electoral interests to do so. To grant him credibility would be to strengthen his hand, which would weaken theirs. So they don’t, no matter how many border guards he hires, no matter how closely his healthcare plan hews to Mitt Romney’s, no matter how many coal subsidies are written into the cap-and-trade bill. They just don’t.

Policy concessions do not generate credibility that can be spent later because credibility is relational, a perception, and in the post-truth era, perceptions are created with the tools of rhetoric and power, not the tools of policy. Creating perceptions means proactively creating them, not making good-faith gestures and waiting, forlorn, for them to be perceived.

Obama has credibility on any number of issues “if only the right would acknowledge it.” But the right won’t, unless they’re forced to by public opinion or demographic changes. Deporting Mexicans won’t do it.