How to Know What's Really There: Robert Boyle and the Art of Experiment

Imagine you’re trying to figure out whether a mysterious powder is safe to eat. You could sit in a chair and think about it, reasoning from first principles about what powders are like. Or you could mix a tiny bit with water, see if it fizzes, maybe feed a little to your friend’s pet hamster (after getting permission). The second approach is messier, slower, and more uncertain—but it’s also how humans actually learned to understand the natural world.

In the 1600s, a brilliant and curious man named Robert Boyle became obsessed with this difference between thinking and testing. He lived at a time when most scholars believed you could figure out how nature works just by sitting and reasoning correctly. Boyle thought that was backwards. He believed you had to get your hands dirty, build strange machines, try things that failed, and only then start making guesses about what’s really going on.

This article is about what Boyle discovered: not just about air and chemistry, but about how we know anything at all.

The Experimenter’s Dilemma

Here’s a problem that might not seem like a problem: How do you know when you’ve proved something?

In geometry, proving something means showing that it must be true. If you prove that the angles of a triangle add up to 180 degrees, you haven’t just found evidence that suggests it—you’ve shown it couldn’t be otherwise. That kind of proof feels solid and final.

But most of what we want to know about the world isn’t like geometry. Does the air have weight? Can matter be divided forever? Is there a God? You can’t just sit and deduce answers to these questions. You have to go out and look—and looking always comes with uncertainty.

Boyle was one of the first people to take this problem seriously. He realized that experiments give us something valuable, but what they give isn’t geometric certainty. It’s something he called moral certainty—the kind of confidence you have when all the evidence points one way, even though you can’t prove it absolutely. In a criminal trial, the jury doesn’t need to be mathematically certain that the defendant is guilty. They need to be sure “beyond reasonable doubt.” That’s moral certainty.

Boyle thought most of science works the same way. Experiments don’t prove things like geometry does. But they can make a conclusion so overwhelmingly likely that it would be unreasonable to doubt it. And that, he argued, is good enough for humans—since we’re not angels with perfect minds.

The Air-Pump and the Law

This might all sound abstract, but Boyle’s most famous discovery shows exactly how his thinking worked.

In the 1650s, Boyle built something remarkable: an air-pump that could suck most of the air out of a glass container. (It leaked constantly and required two men to operate, but it mostly worked.) With this machine, he could study what happens inside a vacuum—something almost nobody had been able to do before.

One thing he noticed was this: If you trap some air in a J-shaped glass tube and pour mercury into the open end, the air gets compressed. And the more mercury you pour in, the more the air squishes down. Boyle measured carefully and found that when he doubled the pressure on the air, its volume was cut in half. Triple the pressure, one-third the volume. The relationship was beautifully simple.

Today we call this Boyle’s Law: for a fixed amount of gas, pressure times volume equals a constant. It’s taught in every chemistry class. But here’s the weird thing: Boyle himself didn’t think he’d discovered a universal law. He was very careful to say that his experiments only showed this relationship for the particular kind of air he tested, under the particular conditions he used. He suspected it might hold more generally, but he refused to claim more than his experiments could support.

This caution wasn’t timidity. It was a philosophical commitment. Boyle believed that the laws of nature aren’t necessary truths like geometry. They’re contingent—they could have been different if God had wanted them different. So you can’t just assume that what works in your lab will work everywhere in the universe. You have to test.

What Are Things Made Of?

Boyle thought about matter, too, and his ideas about it were just as careful and experimental.

The old view, going back to Aristotle, said that everything is made of four elements: earth, air, fire, and water. Different mixtures of these four elements explained why gold is different from wood, or why ice is different from steam. But Boyle wasn’t satisfied with this. He noticed that when you dissolve something in acid and then recover it, it comes back the same stuff. What does that tell you?

He argued that matter is made of tiny particles—corpuscles, he called them—that are too small to see. These corpuscles have basic properties: size, shape, motion. When they combine in different arrangements, they produce all the different substances we see. Gold and wood and ice aren’t different kinds of matter. They’re the same matter arranged differently.

This was revolutionary. Boyle called it the corpuscular philosophy, and it’s basically the ancestor of modern atomic theory. But notice what he was doing: he wasn’t just asserting that matter is corpuscular. He was pointing to experimental evidence. The fact that you can dissolve camphor (a strong-smelling white substance) and then recover it unchanged suggests that its basic particles survive the process. That’s evidence, not just speculation.

Still, Boyle was honest about the limits. Nobody in the 1600s could actually see atoms. So corpuscularianism was, for Boyle, the best explanation of the evidence—not a proven fact. He was comfortable with that.

God, Design, and the Watch

Boyle was also deeply religious, and he thought carefully about how science and religion fit together. Some people in his time thought the new mechanical view of nature—where everything is just particles bumping into each other—would lead to atheism. If the universe is just a machine, who needs God?

Boyle disagreed. He thought studying nature actually supported belief in God. Here’s his argument:

Imagine someone from a faraway country finds a watch washed up on a beach. They’ve never seen a watch before. But by examining its intricate gears and springs, they can tell that it was designed by an intelligent being. It couldn’t have assembled itself by accident.

The universe, Boyle said, is like that watch—only infinitely more complex. The way the parts of an eye work together to produce vision, the way planets move in orderly paths, the way chemicals react in predictable patterns—all of this cries out for explanation. And the best explanation, he thought, is an intelligent designer.

Now, Boyle knew this wasn’t a proof in the geometric sense. It’s what he called a moral demonstration: the conclusion is highly probable, given the evidence, but you can’t force someone to accept it. That’s okay. We make decisions based on probabilities all the time, in courts and in everyday life. Why should the question of God be different?

Some Difficulties

Boyle’s system is elegant, but it runs into problems—and he knew it.

One problem: If God set up the laws of nature and then lets things run on their own, how exactly does God interact with the world? Boyle thought matter has real causal powers—it can push and pull and transfer motion. But he also thought those powers only work because God constantly “concurs” with them. Is that a real explanation, or is it just saying “God does it” without explaining how?

Another problem: If all we can ever have is moral certainty, how do we know when we have enough evidence? When should we stop testing and start believing? Boyle’s answer was basically “use your judgment,” which is honest but not very helpful.

And then there’s the problem of other minds—or in this case, other souls. Boyle believed humans have immaterial souls that can think and perceive, even though the brain is just a machine made of particles. How do the soul and the brain communicate? He couldn’t explain it. He called it supra-mechanical—above the mechanical, not contrary to it. That’s a label, not an explanation.

Why This Still Matters

You might think this is all ancient history. Nobody argues about air-pumps anymore, and we know atoms exist. But Boyle’s deeper questions are still very much alive.

How much evidence is enough? Scientists today argue about this constantly. When can a drug be declared safe? When can we say climate change is real? When is a new theory confirmed? These are Boyle’s questions, dressed in modern clothes.

What’s the relationship between observation and theory? Boyle insisted that you must gather lots of experiments before building theories. Modern science sometimes works that way, and sometimes works backwards—starting with a beautiful theory and then looking for evidence. Philosophers still argue about which approach is better.

And the watchmaker argument? It’s still debated today, in much the same form Boyle gave it. Some people find it convincing. Others point out that evolution by natural selection can explain apparent design without a designer. The argument hasn’t been settled.

Boyle’s great gift was his willingness to live with uncertainty. He didn’t pretend to have answers he didn’t have. He didn’t claim more than his experiments could support. He built careful arguments and then stepped back and said: “This is what I think, but I might be wrong.” That’s a rare and difficult honesty—and it’s as valuable now as it was in 1660.

Key Terms

Term	What it does in this debate
Moral certainty	A level of confidence that’s strong enough to act on, even though it’s not as ironclad as mathematical proof
Corpuscular philosophy	The view that everything is made of tiny particles with basic properties like size, shape, and motion
Experimental philosophy	The approach that says we should gather observations and experiments first, then build theories
Supra-mechanical	Above mechanical explanation—used for things like mind-body interaction that can’t be explained by particles bumping into each other
Moral demonstration	A kind of argument that doesn’t prove its conclusion but makes it overwhelmingly probable

Key People

Robert Boyle (1627–1691): A wealthy Irish scientist who built air-pumps, did groundbreaking chemistry, and argued that experiments—not pure reasoning—are the way to understand nature. He was also deeply religious and tried to show that science supports belief in God.
Francis Bacon (1561–1626): An earlier English philosopher who argued that science should be based on collecting vast amounts of observations before trying to explain them. Boyle followed this method closely.
René Descartes (1596–1650): A French philosopher who thought you could figure out the fundamental nature of reality by reasoning alone. Boyle borrowed some of Descartes’ ideas about matter but rejected his method.

Things to Think About

Boyle said we can have “moral certainty” that a conclusion is true, even without geometric proof. But where do you draw the line? How much evidence is enough before you decide something is true? Can you think of a situation where you might be morally certain but still wrong?
Boyle argued that the complexity of living things is evidence for a designer. But he also thought matter can arrange itself in complex ways through purely mechanical processes (like atoms combining into different substances). At what point does complexity need a designer, and when is natural process enough?
Boyle was very reluctant to generalize his findings. He discovered the inverse relationship between pressure and volume but wouldn’t claim it was a universal law. Is this caution admirable, or does it hold science back? When is it okay to say “this probably works everywhere” without testing everywhere?
Boyle believed humans have immaterial souls that can’t be explained by physics. He called this “supra-mechanical.” But is that a real explanation? Or is it just giving a fancy name to a mystery? Does naming something you can’t explain make it any clearer?

Where This Shows Up

The scientific method today: Every time a scientist says “more research is needed,” they’re echoing Boyle’s caution. When a drug company tests a medicine in thousands of people before declaring it safe, they’re using Boyle’s approach—gathering lots of experiments before making claims.
Courtrooms: The idea of “beyond reasonable doubt” in criminal trials is exactly Boyle’s concept of moral certainty. Lawyers and juries weigh evidence the same way Boyle said scientists should.
Debates about religion and science: The “watchmaker argument” for God’s existence is still used today, and it’s still controversial. Boyle’s version is one of the earliest and most influential.
The philosophy of science: Scientists and philosophers still argue about how much evidence you need before accepting a theory. Boyle was one of the first to make this question central to how science should work.