law of parsimony

noun Philosophy.

a principle according to which an explanation of a thing or event is made with the fewest possible assumptions.



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Origin of law of parsimony

First recorded in 1830–40 Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2020


What is the law of parsimony?

The law of parsimony is a principle that says that the best explanation is the one that requires you to make the fewest possible assumptions about what’s involved. Another way to say this is that the correct explanation or solution is usually the simplest.

Parsimony means extreme frugality, or stinginess, and in this context it refers to being stingy with assumptions (by trying to avoid them). Law is used in the phrase to mean a rule or principle. The law of parsimony is also called Occam’s Razor, the law of economy, and the principle of economy.

The law of parsimony is a general principle of logic, but most often you’ll see it used in discussions of complex scientific concepts, such as the theory of evolution.

Why is law of parsimony important?

Say you make a sandwich, and you leave it out on the table while you go to get something from the other room. When you come back, the plate is still there, but the sandwich is gone. You have a few hypotheses (guesses) about what happened: maybe your roommate took it, or maybe it was your dog. The law of parsimony says that you should choose the explanation that uses the fewest assumptions.

The law of parsimony is traditionally attributed to William of Ockham (or Occam, who Occam’s Razor is named for), an English philosopher and monk in the 1300s, but he wasn’t the first to suggest the principle. Similar ideas were put forth by many people in earlier times, including the ancient Greek philosopher Aristotle. (Hopefully we can assume you’ve heard of him.)

Back to your disappearing sandwich. Possibility A is that your roommate took it. Based on the circumstances, this requires a few assumptions: that your roommate came home, went into the kitchen, and left without you hearing them. Possibility B is that your dog ate it. This requires more assumptions: that your dog woke up, got out of bed, came into the kitchen without you hearing, and got up on the table far enough to get the sandwich without knocking the plate off the table. Now, roommates and dogs are both notorious for stealing food. But the law of parsimony says that since Possibility B requires more assumptions than Possibility A, Possibility A is the better hypothesis. That doesn’t mean that Possibility A is definitely right, and it’s not a substitute for proof, but it does mean that A is the more logical option, given the available information. This is because there are fewer ways it could be wrong.

Most of the time, the law of parsimony is used by people thinking about things that are a lot more complex than the case of the missing sandwich, such as a biologist trying to determine how an animal evolved, or a doctor figuring out the simplest explanation for someone’s health problems. It’s not a way to figure out the ultimate answer to a question, but it is a useful tool for weighing one possibility against another, especially in order to form a guess. (And by the way, don’t give up on solving that sandwich case. For what it’s worth, we know for a fact that your roommate ate your leftover burrito last week.)

Did you know ... ?

William of Ockham went to Oxford University, but he never finished his degree. He was placed under house arrest for disagreeing with church officials before he could!

What are real-life examples of the law of parsimony?

The law of parsimony can be applied to all kinds of situations, but it’s often used by scientists and mathematicians to help them determine which explanation of a big concept or problem is the simplest or most logical.

What other words are related to law of parsimony?

Quiz yourself!

The law of parsimony states that the most preferable hypothesis is the one with how many assumptions?

A. the most
B. the fewest