Occam's Razor

How it works

Named after the 14th-century friar William of Ockham, this principle states: do not multiply entities beyond necessity. In practice, when two explanations account for the same evidence, the simpler one is more likely to be correct — not because the universe prefers simplicity, but because simpler theories have fewer points of potential failure.

Each additional assumption in an explanation is another place where you could be wrong. A theory that requires five assumptions to work has five chances to fail. A theory that requires two has only two.

The razor doesn’t say the simplest explanation is always right. It says the simplest explanation is the best starting point.

Case study: How Barry Marshall proved the medical establishment wrong about ulcers

For decades, the medical consensus was that stomach ulcers were caused by stress, spicy food, and excess acid. The treatment industry was worth billions. The explanation was complex: psychological stress triggered hormonal cascades that increased acid production, which eroded the stomach lining.

In 1982, Australian doctor Barry Marshall proposed a simpler explanation: ulcers were caused by a bacterium, Helicobacter pylori. The medical establishment dismissed him. His theory was too simple. Ulcers were clearly a complex psychosomatic condition.

Unable to get ethical approval for human trials, Marshall drank a petri dish of H. pylori. He developed gastritis within days. He then cured himself with antibiotics. The simpler explanation — infection, not stress — was correct. Marshall won the Nobel Prize in 2005. The complex, sophisticated theory had been wrong. The simple one that fit the evidence was right.

Real-world examples

Medical diagnosis. A patient presents with a headache, fatigue, and mild fever. Complex explanation: a rare autoimmune condition affecting the brain. Simple explanation: they have the flu. A good doctor starts with the flu hypothesis and only escalates if the evidence demands it.

Business. Your product’s sales dropped last month. Complex explanation: a competitor launched a secret campaign, market sentiment shifted, and your brand positioning is misaligned. Simple explanation: you ran out of stock for two weeks. Check the simple things first.

Debugging code. A page isn’t loading. Complex explanation: there’s a race condition in the async rendering pipeline. Simple explanation: there’s a typo in the URL. Check the typo first.

When to use it

Use Occam’s Razor whenever you’re choosing between competing explanations, when you catch yourself building elaborate theories before checking the obvious, and when diagnosing problems — start simple, escalate complexity only when the evidence demands it.

Common mistakes

The most important mistake is treating it as a law rather than a heuristic. Sometimes the complex explanation really is correct — the world is occasionally surprising. Occam’s Razor says start simple, not stay simple. The second mistake is confusing “simple” with “familiar.” An unfamiliar explanation can still be simpler than a familiar complex one.

Try it now

Think of a problem you’re currently trying to solve. Write down your current explanation or theory. Now ask: “What’s the simplest explanation that accounts for all the evidence?” If it’s different from your current theory, what would you need to see to rule it out?

Related models

These models complement Occam's Razor — they address similar situations from different angles.