Donald Trump just took us out of the Paris climate accord for no good reason. I don’t mean that his decision was wrong. I mean, literally, that he didn’t offer any substantive justification for that decision. Oh, he threw around a few numbers about supposed job losses, but nobody believes that he knows or cares where those numbers came from. It was just what he felt like doing.

And here’s the thing: What just happened on climate isn’t an unusual case — and Trump isn’t especially unusual for a modern Republican. For today’s G.O.P. doesn’t do substance; it doesn’t assemble evidence, or do analysis to formulate or even to justify its policy positions. Facts and hard thinking aren’t wanted, and anyone who tries to bring such things into the discussion is the enemy.

Consider another huge policy area, health care. How was Trumpcare put together? Did the administration and its allies consult with experts, study previous experience with health reform, and try to devise a plan that made sense? Of course not. In fact, House leaders made a point of ramming a bill through before the Congressional Budget Office, or for that matter anyone else, could assess its likely impact.

When the budget office did weigh in, its conclusions were what you might expect: If you make huge cuts in Medicaid and reduce subsidies for private insurance — all so you can cut taxes on the wealthy — a lot of people are going to lose coverage. Is 23 million a good estimate of those losses? Yes — it might be 18 million, or it might be 28 million, but surely it would be in that range.