Occam’s razor (2024)

Occam’s razor is a principle often attributed to 14th–century friar William of Ockhamthatsays that if you have two competing ideas to explain the same phenomenon, you should prefer the simplerone.

Many other people before and after the friar, including Albert Einstein and Isaac Newton, have come up with similar rules, but it is generally attributed (via an alternative spelling of the name of the village in which he grew up) to Ockham because he used the principle with such razor-like logic to state, along with other things, that “God’s existence cannot be deduced by reason alone”.

The principle can be applied in many fields of science and logic. If two computer programs do the same job, for example, the shorter one, in which less code can go wrong, is probably preferable. Or if you are a doctor and a patient turns up complaining of a blocked nose, it is more likely they have a common cold than a rare immune-system disorder. As medical students are sometimes told, “When you hear hoof beats, think horses, not zebras”. Or as the US Navy KISS design principle states, “Keep it simple, stupid”.

You can think of it in terms of basic probability theory. All things can be ascribed a probability of happening. As such, any assumptions you add to your theory introduce further possibilities for error, and if an assumption isn’t improving the accuracy of a theory, it just increases the probability the theory is wrong.

It is important to realise, however, that Occam’s razor is more of a logical guideline than a law. It doesn’t prescribe oversimplification, and if a more complex theory is available that better explains the facts, then the more complex theory should be preferred. As is always the case with science, the empirical evidence should win out.

Francis Crick, a co-discoverer of the structure of DNA, warned that the “simplicity and elegance” of Occam’s razor wasn’t well suited to the messy world of biology. Darwinian evolution by natural selection, for example, is a far more complex theory than just saying all animals are the product of a divine creator, but ultimately, it fits the facts as we know them far better.