Cavalcanti is carrying the torch of a tradition that stretches back through a long line of rebellious thinkers who have resisted the usual dividing lines between physics and philosophy. In experimental metaphysics, the tools of science can be used to test our philosophical worldviews, which in turn can be used to better understand science. Cavalcanti, a 46-year-old native of Brazil who is a professor at Griffith University in Brisbane, Australia, and his colleagues have published the strongest result attained in experimental metaphysics yet, a theorem that places strict and surprising constraints on the nature of reality. They’re now designing clever, if controversial, experiments to test our assumptions not only about physics, but about the mind.

While we might expect the injection of philosophy into science to result in something less scientific, in fact, says Cavalcanti, the opposite is true. “In some sense, the knowledge that we obtain through experimental metaphysics is more secure and more scientific,” he said, because it vets not only our scientific hypotheses but the premises that usually lie hidden beneath.

THE DIVIDING LINE between science and philosophy has never been clear. Often, it’s drawn along testability. Any science that deserves its name is said to be vulnerable to tests that can falsify it, while philosophy aims for pristine truths that hover somewhere beyond the grubby reach of experiment. So long as that distinction is in play, physicists believe they can get on with the messy business of “real science” and leave the philosophers in their armchairs, stroking their chins.

As it turns out, though, the testability distinction doesn’t hold. Philosophers have long known that it’s impossible to prove a hypothesis. (No matter how many white swans you see, the next one could be black.) That’s why Karl Popper famously said that a statement is only scientific if it’s falsifiable — if we can’t prove it, we can at least try to disprove it. In 1906, though, the French physicist Pierre Duhem showed that falsifying a single hypothesis is impossible. Every piece of science is bound up in a tangled mesh of assumptions, he argued. These assumptions are about everything from underlying physical laws to the workings of specific measurement devices. If the result of your experiment appears to disprove your hypothesis, you can always account for the data by tweaking one of your assumptions while leaving your hypothesis intact.

Take, for instance, the geometry of space-time. Immanuel Kant, the 18th-century philosopher, declared that the properties of space and time are not empirical questions. He thought not only that the geometry of space was necessarily Euclidean, meaning that a triangle’s interior angles add up to 180 degrees, but that this fact had to be “the basis of any future metaphysics.” It wasn’t empirically testable, according to Kant, because it provided the very framework within which we understand how our tests work in the first place.

And yet in 1919, when astronomers measured the path of distant starlight skirting the gravitational influence of the sun, they found that the geometry of space wasn’t Euclidean after all — it was warped by gravity, as Albert Einstein had recently predicted.

Or did they? Henri Poincaré, the French polymath, offered up an intriguing thought experiment. Imagine that the universe is a giant disk that conforms to Euclidean geometry, but whose physical laws include the following: The disk is hottest in the middle and coldest at the edge, with the temperature falling in proportion to the square of the distance from the center. Moreover, this universe features a refractive index — a measurement of how light rays bend — that is inversely proportional to the temperature. In such a universe, rulers and yardsticks would never be straight (solid objects would expand and shrink with the temperature gradient) while the refractive index would make light rays appear to travel in curves rather than lines. As a result, any attempt to measure the geometry of the space — say, by adding up the angles of a triangle — would lead one to believe that the space was non-Euclidean.

Any test of geometry requires you to assume certain laws of physics, while any test of those laws of physics requires you to assume the geometry. Sure, the disk world’s physical laws seem ad hoc, but so are Euclid’s axioms. “Poincaré, in my opinion, is right,” Einstein said in a 1921 lecture. He added, “Only the sum of geometry and physical laws is subject to experimental verification.” As the American logician Willard V. O. Quine put it, “The unit of empirical significance” — the thing that’s actually testable — “is the whole of science.” The simplest observation (that the sky is blue, say, or the particle is there) forces us to question everything we know about the workings of the universe.

But actually, it’s worse than that. The unit of empirical significance is a combination of science and philosophy. The thinker who saw this most clearly was the 20th-century Swiss mathematician Ferdinand Gonseth. For Gonseth, science and metaphysics are always in conversation with one another, with metaphysics providing the foundations on which science operates, science providing evidence that forces metaphysics to revise those foundations, and the two together adapting and changing like a living, breathing organism. As he said in a symposium he attended in Einstein’s honor, “Science and philosophy form a single whole.”

With the two tied together in a Gordian knot, we might be tempted to throw up our hands, since we can’t put scientific statements to the test without dragging metaphysical statements along with them. But there’s a flipside to the story: It means that metaphysics is testable. That’s why Cavalcanti, who works at the very edges of quantum knowledge, doesn’t refer to himself as a physicist, or as a philosopher, but as an “experimental metaphysicist.”

I MET WITH CAVALCANTI on a video call. With his dark hair pulled back into a bun, he had a brooding look about him, his careful, serious demeanor offset only by a 15-week-old puppy squirming in his lap. He told me how, as an undergraduate in Brazil in the late 1990s, he worked on experimental biophysics — “very wet stuff,” as he describes it, “getting hearts out of rabbits and putting them under [superconducting] magnetometers,” that sort of thing. Though he soon moved on to drier territory (“working in particle accelerators, studying atomic collisions”), the work was still far from the metaphysical questions already lingering in his mind. “I had been told that the interesting questions in foundations of quantum mechanics had all been resolved by [Niels] Bohr in his debates with Einstein,” he said. So he measured another cross section, churned out another paper, and did it all again the next day.