Get to Know Occam's Razor, the 'Only Thing That Distinguishes Science From Religion' (2024)

• In a new book, a genetics professor honors Occam’s razor.
• The principle separates faith and science and has influenced centuries of work.
• Robert Boyle used Occam’s razor to prove facts in a vacuum ... literally.

Occam’s razor is a figure of speech and fundamental idea that helps us remember the value of simplicity. It specifically states that “when you have two competing theories that make exactly the same predictions, the simpler one is the better.”

It's used a lot in the humanities realm as a thought exercise, but its roots—and most important applications—are in science. The concept comes from a real person, William of Ockham, an Englishman who studied and worked in the early 14th century, a time we’ve only recently begun to emphasize was not, so to speak, the “Dark Ages.”

🤯 You love our weird world. So do we. Let's explore it together.

Ockham and his outsize, vital influence on the sciences are the subject of University of Surrey genetics professor Johnjoe McFadden’s new book, Life Is Simple. In it, McFadden follows the full developmental arc of science in the western world and highlights the way Occam’s courage and principles led to almost every major discovery that came after.

“I got interested in Occam’s razor through my system biology work and was intrigued to discover that William of Occam was born in the village of the same name very close to my workplace at the University of Surrey,” McFadden tells Popular Mechanics via email. “When I followed the trail of his most influential idea, his razor of simplicity, although I started thinking that it was just a tool, one of many, of science, [I] became convinced that it IS science—the only thing that distinguishes science from religion, philosophy, superstition, pseudoscience, or mysticism.”

Ockham was a Franciscan monk, but he drew substantial ire for using scientific logic to disprove Thomas Aquinas’s iconic five proofs for the existence of God. Ockham did not insist that faith was foolish, but that it was simply a very separate thing from science, and that it didn’t make sense to mix the two, stating that “God's existence cannot be deduced by reason alone.” At the time, this idea was transgressive enough for him to spend the rest of his life in hiding. But he didn’t stop his important work—he wrote about political issues like natural rights (God-given rights that can never be taken or given away, like liberty) in the years to come. He died in exile in his early 60s in 1347.

Occam’s razor is illustrated in many examples in McFadden’s book, but one of the most striking is the work of Robert Boyle, famous for the eponymous Boyle’s law, which states that the pressure of a gas increases as the size of the container decreases. Before Boyle, scientists (“natural philosophers”) disagreed about what air was made of. But a throughline traces from Ockham to Boyle, using an absence of matter to shock and surprise his colleagues.

First, some backstory. Aristotle had once used logic to disprove the idea of a vacuum in general. (We now know about the vacuum of space, for example, but he didn't.) His proof was considered religiously dangerous, because the establishment insisted to him that God could make a vacuum if he wanted. Aristotle, who believed a vacuum to be logically impossible, didn’t believe God to be beyond logic.

Aristotle’s principle that “nature abhors a vacuum” was taken almost as dogma for centuries after. Before atomic theory—which states that all matter is made up of tiny particles called atoms—people didn’t have a good idea of what matter really was. René Descartes, a French philosopher famous for coining the phrase “cogito ergo sum” (“I think, therefore, I am”) as a distillation of what was demonstrably real in the world, had continued to believe in an explanation of matter dating back to Aristotle: it was all “frenum,” a medium that filled all the seemingly empty space.

It took a mechanical light bulb moment to prove Aristotle wrong. Evangelista Torricelli, an Italian physicist and mathematician, led the way for Boyle by accidentally inventing the barometer while trying to investigate a pumping problem. A barometer works by having a finite amount of liquid, like mercury, that changes volume in coordination with an amount of air. This was the first almost-vacuum and led to a series of subsequent discoveries. By 1654, Boyle had worked out his own rig of tubes and pumps that made an almost complete vacuum inside of a tube.

Here’s where the spectacle comes in:

Into the chamber, Robert inserted a candle and showed that, as the air was pumped out of the chamber, its flame flickered before extinguishing. Boyle and Hooke had demonstrated for the first time in history that fire needs air. They next inserted a watch with a loud tick. When the vessel was full of air the audience could easily hear its ticking but, as the air was evacuated, its ticking grew quieter until it could no longer be heard. The ticking returned only after Hooke allowed air to flow back into the chamber. The pair had demonstrated that sound needs air.

Boyle placed a compass and a magnet in the chamber as well, to show that these magnetic technologies didn’t need air to work. And finally, he demonstrated that a feather and a lead weight fell at exactly the same speed inside the vacuum tube. Not only was a vacuum real, but it helped to confirm that something in the regular air around us was substantial enough to slow down the lighter, higher-surface-area materials like feathers that could “catch” more of it as they fell.

Boyle’s genius didn’t just extend to these dramatic experiments, but the demonstration helped to stoke scientific curiosity toward these brand-new ideas swimming in the collective scientific consciousness at the time—and Boyle was a scientist of the people. He wrote in vernacular English (rather than the traditional Latin of science), and he documented and illustrated his work in great detail so that his experiments could be recreated.

“For this reason Boyle, like Galileo, is a candidate for the title of father of experimental science,” McFadden explains in the book. In his experiments, Boyle carefully ruled out assumptions, one at a time, making them “beyond necessity,” or no longer needed. That is Occam’s razor at its simplest—trimming just one extra element and leaving better science in its place.

One striking thing about the book was how Occam’s razor is used figuratively, versus as the philosophy of William of Ockham himself. We think of Occam’s razor almost more as a way to design things or connect logical dots by using only the simplest possible design or theory, and that is valid and interesting. But for the real scientists in McFadden’s book, their processes often worked more like Coco Chanel’s classic fashion advice: “Before you leave the house, look in the mirror and take one thing off.”

Like a talented stylist or editor, courageous scientists have identified what is redundant ... and promptly scratched it out. McFadden’s book brings this observation to life using two millennia of scientific advancement, never castigating those who were wrong, but instead highlighting how they helped to shape the correct answers that came later.