# Occam's Razor: Why the Simplest Explanation Usually Wins

> Occam's razor says to prefer the explanation with the fewest assumptions. Learn what it really means, where it comes from, examples and where it fails.

Canonical: https://mentarium.io/mental-models/occams-razor.md

Mental model

6 min read By [Nemo](https://mentarium.io/about) Updated October 30, 2026

> **Occam’s razor** is the principle that, when two explanations account for the same facts equally well, you should prefer the one that needs fewer assumptions. It does not say the simplest explanation is always true. It says extra assumptions should be added only when the evidence requires them.

- **Category**: Mental model (reasoning and explanation)
- **Origin**: William of Ockham, 14th century
- **Best used for**: Diagnosing problems and judging competing explanations
- **Related**: Hanlon's razor, base rates, probabilistic thinking

## Ask yourself

- How many separate things have to be true for this explanation to work?
- Is there a more ordinary explanation that fits the same facts?
- Am I adding assumptions just to protect a conclusion I like?
- What is the most common cause of this kind of problem?
- Does the simple explanation actually explain everything, or am I ignoring a fact that doesn't fit?

## What is Occam’s razor?

Occam’s razor is the rule that, among explanations that fit the facts equally well, you should prefer the one with the fewest assumptions. It tells you where to bet first, not what is true.

The reason it works is arithmetic. Every assumption in an explanation is a thing that might be wrong. An explanation that needs five things to be true at once is, almost always, less likely than one that needs two. So when your printer stops working, “the cable came loose” beats “a firmware update interacted badly with the new router and the office VPN,” not because complicated things never happen, but because simple ones happen more often.

Two points get lost in the popular version:

- **“Simplest” means fewest assumptions,** not the easiest to understand or the most comfortable.
- **Both explanations must fit the facts.** A simple story that ignores evidence is not simple, it is wrong.

## Where does Occam’s razor come from?

The principle is named after William of Ockham, an English Franciscan friar and philosopher of the 14th century. Ockham argued repeatedly that one should not assume more than is necessary to explain something, and he used that rule to cut away philosophical entities he considered unneeded.

The famous Latin line, “entities should not be multiplied beyond necessity,” is almost certainly not his. Scholars have not found it in his surviving writings; it was attached to his name by later authors. The idea itself is older still. Aristotle made similar arguments for preferring explanations with fewer premises.

The razor became a working tool of science. Astronomers preferred models of the solar system that explained planetary motion with fewer moving parts. Modern statistics has formal versions of the same idea: a model with more parameters will always fit past data better, so it has to earn those extra parameters by predicting new data better too.

You will also see a line often attributed to Albert Einstein: “Everything should be made as simple as possible, but not simpler.” There is no record of him saying it in that form. The closest documented source is a 1933 lecture in which he made a longer, more technical point along similar lines. The paraphrase is still a good summary of the razor’s limits.

## How to use Occam’s razor

You use Occam’s razor by listing competing explanations, counting the assumptions each needs, and checking the simplest one first. The steps:

1. **Write down every explanation you can think of.** Include the boring ones. People skip the obvious answer because it feels too obvious.
2. **Count the assumptions.** For each explanation, list what must be true for it to work. Don’t count words; count independent claims.
3. **Weight by how common each cause is.** An explanation can have few assumptions and still be rare. Check [base rates](https://mentarium.io/mental-models/base-rates): how often does this kind of cause produce this kind of effect?
4. **Test the leading candidate first.** Cheap tests first. If the cable might be loose, check the cable before you call IT.
5. **Add complexity only when the evidence forces it.** If the simple explanation fails a test, move to the next one. Keep track of the facts it couldn’t explain.
6. **Watch for assumption creep.** If you keep adding “and also” to save your favorite explanation, the razor is telling you something.

Doctors teach a version of this with a saying: when you hear hoofbeats, think horses, not zebras. It is Occam’s razor combined with [probabilistic thinking](https://mentarium.io/mental-models/probabilistic-thinking): the common explanation is the best first guess.

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## 3 real-world examples

### Business

Say sales drop 20% in one month. Theories pile up: a competitor’s new campaign, a change in customer taste, a recession starting, a rumor on social media. Before chasing any of them, check the simple ones. Did the checkout break? Did a payment provider change? Did a big customer delay an order? A single cause with one assumption is worth testing before a story that needs the market, the culture and a competitor to shift at once.

### Personal life

Say a friend hasn’t answered your messages for a week. Your mind offers explanations: they are angry about something you said, they are drifting away, they found new friends. The explanation with the fewest assumptions is usually that they’re busy, tired or bad at texting. That doesn’t mean nothing is wrong. It means asking “busy week?” before you build a theory.

### Tech

Say a website loads slowly for some users. The engineer’s first instinct might be a subtle database lock or a memory leak. Occam’s razor says check the simple causes: one large image, a slow third-party script, a missing cache header. Experienced engineers know complex bugs exist; they also know that most slowness has boring causes, so they rule those out in minutes before spending a day on exotic ones.

## Common mistakes

The most common mistake is using Occam’s razor to dismiss evidence instead of to rank explanations. It is a tiebreaker, not a trump card. Watch for these:

- **Confusing simple with familiar.** “It’s just a coincidence” or “they’re just lazy” may feel simple but can hide many assumptions.
- **Ignoring facts that don’t fit.** If the simple explanation leaves one stubborn fact unexplained, that fact matters. It may point to the real cause.
- **Forgetting that the world can be complex.** Medicine has a counter-saying, Hickam’s dictum: a patient can have more than one disease at once. Sometimes the answer is two causes, not one.
- **Using it to protect a belief.** Some people call their own view “the simple explanation” and the other side’s “too complicated.” Count the assumptions honestly on both sides.
- **Stopping once the simple answer seems likely.** Test it. Likely is not confirmed.

## Occam’s razor vs Hanlon’s razor

Occam’s razor is a general rule for any explanation; [Hanlon’s razor](https://mentarium.io/mental-models/hanlons-razor) applies the same logic to people’s behavior. They are close cousins.

|  | Occam’s razor | Hanlon’s razor |
|---|---|---|
| Rule | Prefer the explanation with fewer assumptions | Don’t assume malice when mistakes explain it |
| Applies to | Any event, system or problem | Other people’s actions |
| Origin | William of Ockham, 14th century | Robert J. Hanlon, 1980 (idea is older) |
| Main use | Diagnosis, science, troubleshooting | Conflict, teamwork, relationships |
| Main risk | Dismissing real complexity | Excusing real bad intent |

Hanlon’s razor is, in a sense, a special case: a malicious plan usually requires more assumptions (motive, planning, secrecy) than a simple mistake.

## The Mentarium take

Occam’s razor is less a rule about truth and more a rule about order. It tells you which explanation to check first. Used that way, it saves enormous amounts of time, because most problems really do have ordinary causes. Used as a final verdict, it becomes an excuse to stop thinking.

The deeper value is self-defense. When you notice yourself adding assumptions to keep a favorite theory alive (“the data is wrong, and also the timing was unusual, and also…”), the razor is flagging [confirmation bias](https://mentarium.io/biases/confirmation-bias) in real time. The more “and also” a story needs, the more you should doubt it.

In the Mentarium Decision Loop (Frame → Map → Stress-test → Decide → Review), the razor helps most in Frame. Before you decide what to do about a problem, make sure you have the problem right. List the explanations, count assumptions, test the cheapest first. Then remember [the map is not the territory](https://mentarium.io/mental-models/map-is-not-the-territory): the simplest model is still a model, and reality is allowed to be messier than your explanation of it.

**Counters** [Confirmation Bias](https://mentarium.io/biases/confirmation-bias)

## FAQ

### What does Occam's razor mean in simple terms?

When several explanations fit the facts, start with the one that needs the fewest extra assumptions. It is a rule for where to look first, not a proof that the simple answer is right.

### Did William of Ockham actually say "entities should not be multiplied beyond necessity"?

Not in those words, as far as scholars can tell. That famous phrasing does not appear in his surviving works and came from later writers. Ockham did express the underlying idea, that you should not assume more than you need to, in several forms.

### Is the simplest explanation always correct?

No. Occam's razor only compares explanations that fit the evidence equally well. If the simple story leaves facts unexplained, a more complex one can be better. Medicine, science and engineering are full of cases where the true cause was complicated.

### What is the difference between Occam's razor and Hanlon's razor?

Occam's razor is general: prefer fewer assumptions. Hanlon's razor is a specific application to human behavior: don't assume malice when mistakes or carelessness explain what happened, since malice usually requires more assumptions.

### Why is it called a razor?

In philosophy, a razor is a rule of thumb that lets you shave away unlikely explanations. Occam's is the most famous, which is why Hanlon's razor and others borrowed the name.

## Related models & biases

[Mental model Hanlon's Razor Hanlon's razor is the rule of thumb "never attribute to malice that which is adequately explained by stupidity." In practice it means that when someone's action hurts you, you should first consider mistakes, ignorance, carelessness or competing priorities before concluding they meant harm. It is a default, not a verdict.](https://mentarium.io/mental-models/hanlons-razor) [Mental model Base Rates and Base Rate Neglect A base rate is how often something happens in a relevant population, before you look at any specific case. Base rate neglect (the base rate fallacy) is ignoring that number in favor of vivid case details. Kahneman and Tversky showed it with the taxicab problem, where most people overweight a witness and ignore how few cabs are Blue.](https://mentarium.io/mental-models/base-rates) [Mental model Probabilistic Thinking Probabilistic thinking is the habit of treating outcomes as likelihoods rather than certainties: estimating how likely each possibility is, updating those estimates as new evidence arrives, and judging decisions by the quality of the reasoning and the odds at the time, not only by how they turned out.](https://mentarium.io/mental-models/probabilistic-thinking) [Mental model The Map Is Not the Territory The map is not the territory is the idea that any representation of reality, whether a model, a plan, a metric, a forecast or a belief, is a simplification and not reality itself. Maps are useful because they leave things out, so good thinkers use them while staying alert to what they leave out.](https://mentarium.io/mental-models/map-is-not-the-territory) [Cognitive bias Confirmation Bias Confirmation bias is the tendency to search for, interpret and remember information in a way that supports what you already believe, while overlooking or discounting evidence against it. It affects how you ask questions, which sources you trust and what you recall later, and it works mostly without you noticing.](https://mentarium.io/biases/confirmation-bias)

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