Whether it's the right amount of vitamin D or the fundamental causes of poverty, bewildering scientific disagreement surrounds us. There's an old joke: ask 10 doctors a question, you'll get 11 answers. Beyond sowing confusion, perpetual disagreement can undermine faith in science. You can almost hear a politician say, "If scientists can't make up their minds, why should I believe anything they say?"

Disagreement is at odds with how we think science works. Evidence "proves" a theory, it "shows" us how the world is. Science is supposed to be objective, and scientists follow the evidence wherever it leads. If scientists can disagree for years on end, what does this mean for the objectivity of science?

But while it might feel like a modern phenomenon, scientific disagreement is nothing new. At the beginning of the 19th century, the English chemist, John Dalton, proposed that all matter was made up of tiny atoms. Like so many advances, this idea is much older. Around the 4th century B.C., Democritus—the "laughing philosopher" —proposed the same. Unlike Democritus, however, Dalton brought to bear a substantial body of evidence for his theory.

Chemists were quick to adopt Dalton's ideas. Less than a century after his proposal, chemistry relied on atoms. By 1900, while chemists believed atoms were facts of nature, physicists remained skeptical. Influenced by philosophical theories about scientific knowledge, many thought that the existence of atoms needed further proof.

While physicists knew that modern chemical theory relied on atoms, they imagined that atoms might be merely a "useful fiction" without physical reality. Physicists didn't think that there was evidenceagainstatoms, per se. They merely wanted more evidence before they made up their minds. (Of course, we know that the chemists were right. Spoiler alert: atoms exist.)

Physicist-turned-philosopher Thomas Kuhn suggested one explanation for scientific disagreements like this one. While we talk about scientific theories as "proven" or "unproven," in reality, science is far more complicated. One theory may be better in some ways; it might be easier to work with, for example. A competing theory might be better in another way; perhaps it can predict more accurately. Different scientists might care about one aspect more than another. As a result, they might come to disagree about which theory is better.

You're probably already familiar with the experience. You chose one credit card because the customer service is exceptional. A friend prefers a different card because of its rewards. Neither you nor your friend is "correct" about which card is objectively better; you just care about different things. To each their own.

Kuhn never really fleshed out what the criteria were. Since then, philosophers have filled the gap and come up with a list of the criteria that scientists use. They include criteria you might expect, like the ability to accurately predict phenomena and simplicity of a theory. But they also include others you haven't thought of, like the ability to unify apparently disparate parts of nature, or even aesthetic criteria like mathematical beauty. Some philosophers have even suggested that scientists should add new ones or stop caring so much about others.

Kuhn's differing criteria idea makes sense of the fight between chemistry and physics. The progress in chemistry in the 19th century depended on the atomic theory. Without atoms, understanding a chemical reaction would be difficult, maybe impossible. Unsurprisingly, chemists were strongly motivated by this consideration. The 19th century physicists on the other hand, prizeddirectevidence—the kind they had in other areas of physics. The chemists hadn't seen atoms, nor had they done anything to measure them directly. Without that, the physicists claimed, we were still unsure about whether atoms existed. The physicists prized direct evidence more.

With the credit cards, nothing will resolve your "disagreement" with your friend. In science, we keep exploring, hoping we can satisfy everyone's standards. In the early 1900s, Albert Einstein did just that. He calculated exactly how a microscopic, but visible, particle should behave if it was bumping into unseen atoms—a phenomenon called Brownian motion. Jean Baptiste Perrin performed the experiment that Einstein proposed, and he saw the predicted motion. Taken together Einstein and Perrin, along with several other experiments, convinced physicists that atoms are real.

They convincedalmosteveryone. One physicist, Ernst Mach (after whom the speed Mach 1 is named), remained skeptical. Mach's recalcitrance shows us that we should never ask for complete agreement; some scientists will always be stubborn. Once a vast majority of scientists agree, we've probably demonstrated something real. One or two holdouts don't mean much.

Physicists didn't come around because they started caring more about chemistry. Rather, the two different criteria pointed in the same direction. Einstein and Perrin gave physicists what they wanted (and what the chemists thought was unnecessary). It didn't matter what you cared about; either way you believed in atoms.

Today there is disagreement over the value of vitamin D, the causes of poverty and much more. Even where there is some substantial agreement, like on climate change, much debate remains. Science encourages both agreement and disagreement. Like with atoms, as evidence builds, we eventually find a way to agree.

It's easy to overemphasize the problem of disagreement in science. While climate scientists disagree about many things, there is broad consensus on one major point: that humans are substantially altering the climate. While smart people may disagree about details—like exactly how much the earth will warm—there is much they agree on.

But when a substantial number of scientists fundamentally disagree, we just have to wait. Happily, the disagreements usually sort themselves out. Until then you—like the chemists and the physicists of the early 20th century—can feel comfortable in the knowledge that no matter which side you take you're in good company.