Occam’s razor: the simpler explanation is usually the right one

Occam’s razor is a way of making logical decisions. It tells us that if we have two different explanations for one phenomenon, we should prefer the simpler one.

The razor is named after the English philosopher and theologian William of Ockham, who lived in the fourteenth century. William of Ockham himself formulated the principle in several different ways, but the formulation that caught on most is “entities should not be multiplied beyond necessity”, which was devised by the Irish philosopher John Punch.

What Occam’s razor tells us

Imagine that something has happened in your town. Say someone has set fire to a tree stump in the market square. The police begin to investigate the case with their usual zeal and catch the culprit within a few hours. In court, three witnesses describe how they saw the culprit set fire to the stump. One child even filmed it and posted it on TikTok. The culprit’s wife also confirms that at the time in question he was not at home and had gone somewhere. The case seemed clear-cut.

The culprit, let’s call him Harry, defended himself with an unusual argument. He claimed that he had not done it, but that it had been done by a double, a clone of him built by the evil demon Pazuzu in an attempt to enslave humanity, and that setting fire to the stump was the first phase of his evil plan.

“Prove to me that it isn’t so,” said Harry confidently.

You have probably guessed the problem already. Proving that there is no double and no demon Pazuzu is impossible. In purely theoretical terms, it could really have happened that the demon Pazuzu rose from somewhere and created Harry’s double, who set fire to a stump in Leeds, that is, in your town, and then vanished again. It simply isn’t possible to rule it out one hundred per cent. So how should the judge decide?

At this point the judge can use Occam’s razor. It says that if we have two different explanations that account for a phenomenon equally well, we should use the simpler explanation. In our case, the explanation that Harry is lying and set fire to the stump is far simpler than that a mysterious demon Pazuzu rose from hell.

In fact, we don’t have just two different explanations. Harry can come up with infinitely many others: the stump was set on fire by angels, who also bewitched all the witnesses; the stump didn’t actually burn and it is just a group hallucination; or someone simply bribed all the witnesses and faked the video recording, and so on. But the simplest explanation is that Harry simply set it on fire.

What Occam’s razor does not tell us

Occam’s razor does not tell us that we have decided correctly, though! It really could have happened that someone bribed all the witnesses or that angels descended from heaven to earth. We don’t know. Occam’s razor only advises us which explanation has the highest probability. If the judge did not apply Occam’s razor, every criminal would walk out of court as an innocent person, because each of them could invent their own story with their own demon Pazuzu, which in fact cannot be refuted.

We can therefore also define Occam’s razor in terms of how hard it is to refute an explanation of a given phenomenon. If we have two explanations of a phenomenon, we would rather choose the one that is easier to refute. There are many ways to refute that Harry is the real culprit. For example, someone might have seen him on the other side of town. Or cameras might have caught him on that other side of town. A petrol can with someone else’s fingerprints might have been found at the scene. And so on. How would we refute that the demon Pazuzu descended to earth? Well, that is hard to do. Since it is practically impossible to refute this, we prefer the explanation that Harry did it, because it is much easier to refute and yet nobody has refuted it.

The harder an explanation is to refute, the more complicated an explanation it is and the less probable it is.

Occam’s razor in science

Of course, the razor is widely used in science, where we have to apply it at every step. For example, why do things fall to the ground? We can either attribute it to gravity, or we can say that the moment you drop an object, invisible flying dwarfs immediately arrive and start pushing the object until it lands on the ground.

At this point we can apply Occam’s razor and say that it is more likely to be gravity than dwarfs.

Simpler is not always more correct

The motion of bodies is described by two fundamental theories: Newton’s laws of motion and the theory of relativity by Albert Einstein. While Newton’s laws of motion are taught at secondary school, the theory of relativity is much, much more complicated. According to Occam’s razor, we should at this point discard Einstein’s theory of relativity and keep following Newton’s laws.

That is not so, for one simple reason. The two theories do not provide equally good explanations of phenomena in nature. The theory of relativity provides a much more accurate description of motion in our universe. The closer the speed of an object gets to the speed of light, the less accurate Newton’s laws of motion become. For ordinary speeds on Earth, Newton’s laws are perfectly sufficient, but as soon as you want to calculate the motion of satellites around our Earth, you need the theory of relativity, which is more complicated by orders of magnitude, but also much more accurate. If, for example, the GPS positioning system used only Newton’s laws of motion, before long your GPS would be showing your position several kilometres away from where you really are.

So in this case it is not possible to use Occam’s razor, because the theory of relativity is more complicated, but also more accurate. The two do not describe a phenomenon equally well. Einstein’s theory describes it much better. It is like comparing the watch you wear on your wrist with the atomic clocks used by scientists. Of course atomic clocks are much more complicated, but also much more accurate. So they do not describe the same phenomenon equally well, and that is why we cannot apply Occam’s razor.