What was President Trump up to when he tweeted that the concept of global warming was invented by the Chinese to make US manufacturing less competitive? What do pro-Leave campaigners hope to achieve by describing warnings about the economic impact of Brexit as “Project Fear”? Both are examples of a popular and highly potent political tactic: rubbishing.

Rubbishing is a more extreme version of the so-called “Tobacco Strategy.” In their book Merchants of Doubt, Naomi Oreskes and Erik Conway describe the reaction of cigarette companies in the 1950s to the discovery that their product is a killer: They hired experts to question the link between smoking and lung cancer. Strong evidence of a link was dismissed as inconclusive, as smokers were encouraged not to give up.

The Tobacco Strategy was an exercise in fact-fighting. The point was to create doubts in the minds of smokers about whether smoking was really bad for them. In the words of a notorious memo written by a company executive, “Doubt is our product.”

Raising questions about a piece of evidence is one thing; rubbishing it is another. The point of Trump’s tweet was to rubbish evidence of climate change caused by humans. The point of the Project Fear label is to rubbish warnings by economists and business that Brexit is going to be bad for the economy.

But even though rubbishing is more extreme than the Tobacco Strategy the basic objective is the same: to make it harder for non-experts to know the facts. If we don’t really know that humans are responsible for climate change, why do anything about it? Why fear Brexit if warnings about its economic impact are rubbish?

The key to rubbishing is the connection between knowledge and confidence. For you to know that smoking is bad for you it’s not enough to think you know it’s bad for you. You also need to be reasonably confident that it’s bad, and you need to have the right to be confident. To know that humans are responsible for climate change you have to be reasonably confident that your view is correct. If that’s right, then undermining a person’s confidence is a way to undermine their knowledge.

Knowledge is based on evidence but most of us don’t have direct evidence for many-widely accepted theories. Unless you are a climate scientist, you are unlikely to have detailed knowledge of the evidence for climate change. Much of our knowledge is what philosophers call “testimonial” knowledge. It’s knowledge based on what other people tell us. Taking another person’s word for it is justified only if they know what they are talking about and you have good reason to trust them.

Imagine a smoker who is reasonably confident that smoking is bad for him and resolves to give up. By shaking his confidence in the dangers of smoking, the Tobacco Strategy can deprive him of his knowledge that smoking is bad. That’s what happens when, as a result of the Tobacco Strategy, a smoker goes from knowing that smoking is bad to not being sure whether it is. In the same way, sceptical doubts about the reality of climate change can deprive a non-expert of the confidence that climate change is real.

Rubbishing is not the only way of depriving a person of their knowledge. Instead of directly attacking the evidence for a claim one might instead choose to attack the experts responsible for bringing the evidence to light. Anti-expertism is part and parcel of the post-truth agenda. Its point is to allow the public to ignore inconvenient truths by implying that those responsible for uncovering them aren’t to be trusted.

Just for good measure it’s always possible to add an element of conspiracy theory. That’s the point of Trump’s tweet, to suggest that a Chinese conspiracy is responsible for the idea that humans are responsible for climate change. Gaslighting is another method. It’s hard to know whether climate change is real or Brexit will be bad for the economy if you don’t trust yourself to know. Damaging a person’s confidence is a way to damage their knowledge.

It’s always possible to ignore Trump’s tweets or the Tobacco Strategy. We tend to think of open-mindedness as an intellectual virtue and closed-mindedness as an intellectual vice. But isn’t it sometimes good to be closed-minded? Why should one’s mind be open to the claims of climate change deniers or other attempts to rubbish established knowledge?

The correct response to rubbishing can be to engage in some rubbishing of one’s own. Where a piece of rubbishing—like Trump’s tweet—is obviously itself rubbish it’s legitimate to point that out. But that doesn’t always work. Sometimes one doesn’t have the right to be confident in one’s beliefs unless one can defend them. If you have no answer to the rubbishers, how can you be entitled to hold on to your beliefs? How can you really know?

This makes it sound like you need to find out for yourself what the facts are—but which of us has the time or expertise to do that? In practice there are shortcuts. For example, it’s not that hard to find out that the Chinese didn’t come up with the idea of climate change and that the overwhelming majority of climate scientists concur that humans are responsible for climate change. Some of the most prominent deniers, like Senator Inhofe of Oklahoma, aren’t scientists but politicians. Is it so hard to figure out whether to believe a partisan politician or a bunch of accredited scientists?

The same goes for Project Fear. Not attaching much weight to the pronouncements Boris Johnson or Nigel Farage isn’t closed-minded or dogmatic. It’s a legitimate reaction based on an objective assessment of their reliability. Our knowledge is fragile, and the only way to protect it is to take on and defeat the rubbishers.

Vices of the Mind: From the Intellectual to the Political is published by Oxford University Press on 14th February.