Guide to Occam’s Razor | Definition and Example

Guide to Occam's Razor | Definition and Example - Fallacy in Logic

Occam’s razor, also known as the law of parsimony, is a principle that is intended to simplify decision-making process in certain circumstances. It’s a type of mental model, meaning a particular way to think about a complex issue in order to make it easier to solve.

Moreover, the term “razor” refers to a philosophical tool that helps shave off unlikely explanations and makes decision-making more effective in a given situation. Occam’s razor is probably the most common one, but there are a number of others, such as Hanlon’s razor.


Occam’s razor states that:

  • Entities should not be multiplied without necessity.

Depending on the source, this is also expressed as:

  • Among competing hypotheses, the one with the fewest assumptions should be selected.

In other words, it essentially says that simpler explanations should be preferred over their more complex alternatives.

It suggests that, instead of making issues excessively complicated, we should prune off excess assumptions and concentrate on the solutions that are controllable and evidently more likely to work. Or, at least, if you already have something that works, don’t try to make it more complex without very good reasons.

It can be employed as a guiding principle in a range of circumstances where one needs to make the most efficient decision while lacking empirical evidence.


A good example of the principle comes from the field of medicine. In today’s treatment of patients, doctors are advised to narrow down the likely causes to the smallest possible number and always prefer the most likely options.

This means, for example, that if a patient is showing all the symptoms of a common cold, it is more reasonable to assume that they are suffering from a common cold than from something similar but less common illness (even though it may be reasonable or necessary to examine other options too).


Guide to Occam's Razor | Definition and Example - Fallacy in  Logic

There are limitations to Occam’s razor, just like to any other philosophical razor or mental model. Thus, it should always be applied with appropriate care.

Firstly, note that it doesn’t claim that the simplest answer is necessarily the correct one. Instead, it suggests that in a case where there are two or more competing solutions to a problem, the safest bet is often the one with fewer assumptions.

Accordingly, this principle should not be applied when it goes against logic and facts; if the evidence clearly points in a different direction, it would be unreasonable to dismiss it in favor of the simpler explanation. As such, the razor should be regarded as a useful tool for making effective decisions in a given situation, not as a substitution for the whole process of reaching the correct conclusion based on facts and sound reasoning. 


Occam’s razor is attributed to William of Ockham, a 14th-century scholar, who stated: pluralistas non est ponenda sine necessitate. Translated to English, this means “plurality should not be posited without necessity.” He employed it, for instance, to justify the conclusion that “God’s existence cannot be deduced by reason alone.”

Although the razor is dubbed after William, he was not the first person to use or write about it. In fact, it has been stated, in its various but closely related forms, by several people in different stages of history.

Aristotle (384 – 322 BC), one of the best-known philosophers of all time, touched on it when he wrote in his Posterior Analytics that “we may assume the superiority, other things being equal, of the demonstration which derives from fewer postulates or hypotheses”.

Claudius Ptolemy (90 – 168 AD), a philosopher, mathematician, astronomer, and astrologer, also stated “we consider it a good principle to explain the phenomena by the simplest hypothesis available.”

Furthermore, theologian and philosopher Durandus of Saint-Pourcain (1275 – 1334) used it to explain how abstraction results from the fear of some real entity.