It’s often assumed that physics and philosophy are at opposite ends of the academic spectrum. In fact, they’re close—so close that they can overlap, with professors sometimes switching between the two fields as they work to advance our understanding of highly abstract subjects in theoretical physics.

One such professor is Richard Dawid, a philosophy of science researcher at Ludwig Maximilian Universität Munich, who has a PhD in theoretical physics and began his career researching particle physics. He transitioned to philosophy, he tells Quartz, to investigate how physicists can come to believe in certain theories without necessarily having the empirical evidence that proves them.

The criteria for establishing a theory, he discovered, is not in itself subject to scientific enquiry. “They’re considered background assumptions,” says Dawid. “It’s a question that’s driven by physics but it’s a philosophical question.”

Physicists have long relied on a notion advanced by philosopher Karl Popper, that a theory is scientifically valid if it is falsifiable. But in recent years, many serious physicists seem to have abandoned this model. String theory, for example, is one of the most exciting ideas in modern physics. But it’s not testable—so how can physicists be confident that it’s sound?

String theory is at the crux of physics, as it links quantum mechanics with Albert Einstein’s theory of relativity. The theory argues that subatomic particles are tiny, one-dimensional strings, rather than zero-dimensional points, and are constantly vibrating. It’s thought that at least 10 dimensions are curled up within the strings—but the particles themselves are too small to be detected, let alone these extra dimensions.

“String theory is not a fully developed theory,” Dawid tells Quartz. “Some predictions string theory may give cannot be calculated. It’s not accessible by present-day experiments or experiments in the foreseeable future. So what can we do with a theory of that kind? We don’t want to live with a theory for 50 years and have no clue whether it’s viable or not.”

The implausibility of testing string theory not just now, but for decades to come, is a new problem for physics. Although Einstein never conducted a single experiment, he put forward his theory in 1915 and, by 1919, experiments found evidence to support one of his key predictions, that a gravitational field causes light to bend.

In cases where empirical evidence is unavailable, Dawid has identified three reasons why people believe in the veracity of the theoretical physics model; none of them are particularly strong by themselves but, in conjunction, he believes that they present strong grounding for trusting an un-provable theory.

The first is the no-alternative argument: “Even though people have tried to find possible solutions to the problem, only one has been convincing,” Dawid says.

The second is the meta-inductive argument, whereby if a theoretical research program has previously come up with a theory that was later proven, later theories have a similarly good chance of being verified; in this case, Dawid believes that string theory research is comparable to standard-model research. “In high energy physics, whenever a solution emerged that was the only one that could be found, chances were very high that eventually that theory was verified with empirical evidence,” he says.

And finally, there’s the unexpected explanatory coherence argument, where a theory solves physics questions beyond those it was intended to solve. “The theory gives you more than you planned to get out from it,” says Dawid.

Even taken together, these arguments are not as strong as empirical evidence, Dawid notes. But they are better than other reasons physicists might put forward for supporting a certain model, such whether or not a theory is beautiful. “Arguments about beauty or elegance can be used as an emotional background but they’re not helpful for getting an understanding about whether your theory is viable,” he says.

Not all physicists believe that Dawid has identified sound reasons for trusting a scientific theory, but the subject has started to be seriously debated by physicists and was the focus of a December conference at LMU Munich, were Dawid spoke.

Physicists aren’t as philosophically engaged as they once were—in the early 20th century, physicists such as Henri Poincaré thought it was important to develop their philosophical conceptions of their scientific work. But as the methodology in physics becomes increasingly more abstract and less stable, there’s a growing interest among physicists in what philosophy has to offer. “Physicists are once again more willing to consider philosophical arguments,” says Dawid.