Last week Eric Cantor, the House majority leader, gave what his office told us would be a major policy speech. And we should be grateful for the heads-up about the speech’s majorness. Otherwise, a read of the speech might have suggested that he was offering nothing more than a meager, warmed-over selection of stale ideas.

To be sure, Mr. Cantor tried to sound interested in serious policy discussion. But he didn’t succeed — and that was no accident. For these days his party dislikes the whole idea of applying critical thinking and evidence to policy questions. And no, that’s not a caricature: Last year the Texas G.O.P. explicitly condemned efforts to teach “critical thinking skills,” because, it said, such efforts “have the purpose of challenging the student’s fixed beliefs and undermining parental authority.”

And such is the influence of what we might call the ignorance caucus that even when giving a speech intended to demonstrate his openness to new ideas, Mr. Cantor felt obliged to give that caucus a shout-out, calling for a complete end to federal funding of social science research. Because it’s surely a waste of money seeking to understand the society we’re trying to change.

Want other examples of the ignorance caucus at work? Start with health care, an area in which Mr. Cantor tried not to sound anti-intellectual; he lavished praise on medical research just before attacking federal support for social science. (By the way, how much money are we talking about? Well, the entire National Science Foundation budget for social and economic sciences amounts to a whopping 0.01 percent of the budget deficit.)

But Mr. Cantor’s support for medical research is curiously limited. He’s all for developing new treatments, but he and his colleagues have adamantly opposed “comparative effectiveness research,” which seeks to determine how well such treatments work.

What they fear, of course, is that the people running Medicare and other government programs might use the results of such research to determine what they’re willing to pay for. Instead, they want to turn Medicare into a voucher system and let individuals make decisions about treatment. But even if you think that’s a good idea (it isn’t), how are individuals supposed to make good medical choices if we ensure that they have no idea what health benefits, if any, to expect from their choices?

Still, the desire to perpetuate ignorance on matters medical is nothing compared with the desire to kill climate research, where Mr. Cantor’s colleagues — particularly, as it happens, in his home state of Virginia — have engaged in furious witch hunts against scientists who find evidence they don’t like. True, the state has finally agreed to study the growing risk of coastal flooding; Norfolk is among the American cities most vulnerable to climate change. But Republicans in the State Legislature have specifically prohibited the use of the words “sea-level rise.”

And there are many other examples, like the way House Republicans tried to suppress a Congressional Research Service report casting doubt on claims about the magical growth effects of tax cuts for the wealthy.

Do actions like this have important effects? Well, consider the agonized discussions of gun policy that followed the Newtown massacre. It would be helpful to these discussions if we had a good grasp of the facts about firearms and violence. But we don’t, because back in the 1990s conservative politicians, acting on behalf of the National Rifle Association, bullied federal agencies into ceasing just about all research into the issue. Willful ignorance matters.

O.K., at this point the conventions of punditry call for saying something to demonstrate my evenhandedness, something along the lines of “Democrats do it too.” But while Democrats, being human, often read evidence selectively and choose to believe things that make them comfortable, there really isn’t anything equivalent to Republicans’ active hostility to collecting evidence in the first place.

The truth is that America’s partisan divide runs much deeper than even pessimists are usually willing to admit; the parties aren’t just divided on values and policy views, they’re divided over epistemology. One side believes, at least in principle, in letting its policy views be shaped by facts; the other believes in suppressing the facts if they contradict its fixed beliefs.

In her parting shot on leaving the State Department, Hillary Clinton said of her Republican critics, “They just will not live in an evidence-based world.” She was referring specifically to the Benghazi controversy, but her point applies much more generally. And for all the talk of reforming and reinventing the G.O.P., the ignorance caucus retains a firm grip on the party’s heart and mind.