The following was delivered as the commencement address at the California Institute of Technology, on Friday, June 10th.

If this place has done its job—and I suspect it has—you’re all scientists now. Sorry, English and history graduates, even you are, too. Science is not a major or a career. It is a commitment to a systematic way of thinking, an allegiance to a way of building knowledge and explaining the universe through testing and factual observation. The thing is, that isn’t a normal way of thinking. It is unnatural and counterintuitive. It has to be learned. Scientific explanation stands in contrast to the wisdom of divinity and experience and common sense. Common sense once told us that the sun moves across the sky and that being out in the cold produced colds. But a scientific mind recognized that these intuitions were only hypotheses. They had to be tested.

When I came to college from my Ohio home town, the most intellectually unnerving thing I discovered was how wrong many of my assumptions were about how the world works—whether the natural or the human-made world. I looked to my professors and fellow-students to supply my replacement ideas. Then I returned home with some of those ideas and told my parents everything they’d got wrong (which they just loved). But, even then, I was just replacing one set of received beliefs for another. It took me a long time to recognize the particular mind-set that scientists have. The great physicist Edwin Hubble, speaking at Caltech’s commencement in 1938, said a scientist has “a healthy skepticism, suspended judgement, and disciplined imagination”—not only about other people’s ideas but also about his or her own. The scientist has an experimental mind, not a litigious one.

As a student, this seemed to me more than a way of thinking. It was a way of being—a weird way of being. You are supposed to have skepticism and imagination, but not too much. You are supposed to suspend judgment, yet exercise it. Ultimately, you hope to observe the world with an open mind, gathering facts and testing your predictions and expectations against them. Then you make up your mind and either affirm or reject the ideas at hand. But you also hope to accept that nothing is ever completely settled, that all knowledge is just probable knowledge. A contradictory piece of evidence can always emerge. Hubble said it best when he said, “The scientist explains the world by successive approximations.”

The scientific orientation has proved immensely powerful. It has allowed us to nearly double our lifespan during the past century, to increase our global abundance, and to deepen our understanding of the nature of the universe. Yet scientific knowledge is not necessarily trusted. Partly, that’s because it is incomplete. But even where the knowledge provided by science is overwhelming, people often resist it—sometimes outright deny it. Many people continue to believe, for instance, despite massive evidence to the contrary, that childhood vaccines cause autism (they do not); that people are safer owning a gun (they are not); that genetically modified crops are harmful (on balance, they have been beneficial); that climate change is not happening (it is).

Vaccine fears, for example, have persisted despite decades of research showing them to be unfounded. Some twenty-five years ago, a statistical analysis suggested a possible association between autism and thimerosal, a preservative used in vaccines to prevent bacterial contamination. The analysis turned out to be flawed, but fears took hold. Scientists then carried out hundreds of studies, and found no link. Still, fears persisted. Countries removed the preservative but experienced no reduction in autism—yet fears grew. A British study claimed a connection between the onset of autism in eight children and the timing of their vaccinations for measles, mumps, and rubella. That paper was retracted due to findings of fraud: the lead author had falsified and misrepresented the data on the children. Repeated efforts to confirm the findings were unsuccessful. Nonetheless, vaccine rates plunged, leading to outbreaks of measles and mumps that, last year, sickened tens of thousands of children across the U.S., Canada, and Europe, and resulted in deaths.

People are prone to resist scientific claims when they clash with intuitive beliefs. They don’t see measles or mumps around anymore. They do see children with autism. And they see a mom who says, “My child was perfectly fine until he got a vaccine and became autistic.”

Now, you can tell them that correlation is not causation. You can say that children get a vaccine every two to three months for the first couple years of their life, so the onset of any illness is bound to follow vaccination for many kids. You can say that the science shows no connection. But once an idea has got embedded and become widespread, it becomes very difficult to dig it out of people’s brains—especially when they do not trust scientific authorities. And we are experiencing a significant decline in trust in scientific authorities.

The sociologist Gordon Gauchat studied U.S. survey data from 1974 to 2010 and found some deeply alarming trends. Despite increasing education levels, the public’s trust in the scientific community has been decreasing. This is particularly true among conservatives, even educated conservatives. In 1974, conservatives with college degrees had the highest level of trust in science and the scientific community. Today, they have the lowest.

Today, we have multiple factions putting themselves forward as what Gauchat describes as their own cultural domains, “generating their own knowledge base that is often in conflict with the cultural authority of the scientific community.” Some are religious groups (challenging evolution, for instance). Some are industry groups (as with climate skepticism). Others tilt more to the left (such as those that reject the medical establishment). As varied as these groups are, they are all alike in one way. They all harbor sacred beliefs that they do not consider open to question.

To defend those beliefs, few dismiss the authority of science. They dismiss the authority of the scientific community. People don’t argue back by claiming divine authority anymore. They argue back by claiming to have the truer scientific authority. It can make matters incredibly confusing. You have to be able to recognize the difference between claims of science and those of pseudoscience.

Science’s defenders have identified five hallmark moves of pseudoscientists. They argue that the scientific consensus emerges from a conspiracy to suppress dissenting views. They produce fake experts, who have views contrary to established knowledge but do not actually have a credible scientific track record. They cherry-pick the data and papers that challenge the dominant view as a means of discrediting an entire field. They deploy false analogies and other logical fallacies. And they set impossible expectations of research: when scientists produce one level of certainty, the pseudoscientists insist they achieve another.

It’s not that some of these approaches never provide valid arguments. Sometimes an analogy is useful, or higher levels of certainty are required. But when you see several or all of these tactics deployed, you know that you’re not dealing with a scientific claim anymore. Pseudoscience is the form of science without the substance.