# Critical Thinking Toolkit: Ockham’s Razor

When we are in debates, very often there is more than one way to explain something. When we are presented with more than one way of explaining some aspect of reality – be it scientific, historical, religious, or anything else – these situations arise. When they do, we want to be able to differentiate between the various alternatives in a reasonable way. But it is not immediately obvious how to do something like this. If Steve and Carol provide two totally different explanations for the same event, how do we analyze which one of them is more likely right? After all, we cannot evaluate the alternatives based on their conclusions, since the conclusions are identical. What then do we do?

Ockham’s Razor to the Rescue

To give some kind of intuitive explanation of the principle of reasoning that I will define here, let’s use an example. Suppose that we are tasked with explaining why there are Christmas presents under a Christmas tree. There are two possible answers available to us. For us, Explanation 1 is that Santa Claus lives in the North Pole and delivers presents to all children throughout the world on the night before Christmas morning every year. Explanation 2 is that it is actually the parents of individual children that deliver the presents, and that the story of Santa Claus is a fictional tale meant to inspire fun and imagination in children. Notice that Explanations 1 and 2 both completely describe why there are Christmas presents under the tree, so we cannot evaluate between 1 and 2 based upon which one leads to the correct conclusion (of course we could bring in other information that shows us why Santa can’t exist as described here, but for the sake of argument we will pretend we don’t know any of that). Well then, how might we choose which of the two is more likely true?

This where the principle I want to talk about here comes in. It is not meant to be an absolutely foolproof method, but it is quite reliable. The method, called Ockham’s Razor (Ockham can also be spelled Occham or Ocham) is summarized by the statement “entities should not be multiplied without necessity.” To be more specific, Ockham’s Razor tells us that if you have different ways of explaining exactly the same thing, you ought to take the explanation that makes the fewest assumptions. In the previous example, we already know that our parents exist, so the assumptions involved in explaining Christmas presents via our parents are extremely few – the only assumption we have to add is that our parents are lying to us (if we neither believe nor disbelieve in Santa, that’s really all we need). In order to explain Christmas presents by Santa visiting our house, we have to add significantly more assumptions. Therefore, unless we have new evidence that points strongly one way or the other, Ockham’s Razor tells us to accept the alternative with fewer assumptions – namely, that Santa does not exist.

How to (and not to) Use Ockham’s Razor

Ockham’s Razor is a frequently used and very important tool in critical thinking. I must make that very clear – it is a great principle and we really must take advantage of this way of thinking about complicated issues. But, it is also very necessary to emphasize both the powers and limitations of Ockham’s Razor.

Ockham’s Razor is not All-Powerful: One extremely important reality we have to acknowledge is that this principle does not always work. Ockham’s Razor is meant to ‘shave off’ additional assumptions only if those additional assumptions are not helpful in other ways. Ockham’s Razor favors simple explanations over more complicated explanations, but only when the two complicated explanation doesn’t override the simple explanation in other ways. For example, Isaac Newton’s theory of gravity is much simpler than Albert Einstein’s theory of gravity (aka general relativity). Since the two theories analyze the same concept – namely gravity – they can be compared. If that were the only information we knew, we would have to prefer Newtonian gravity to general relativity. But scientists use general relativity today in the most important applications, not Newtonian gravity. Why? Because general relativity is more accurate than the Newtonian theory. The increase in accuracy is more important than the simplicity. So, if we want to use Ockham’s Razor, we should make sure that the two ideas in question are similar in other respects.

When Occham’s Razor Applies, It Almost Always Works: When you look throughout your own life or the history of any discipline of study in human history, I am quite confident that whenever you find a proper situation in which to apply Ockham’s Razor, it will work correctly. In other words, reality tends to favor simple explanations over complicated ones whenever a simple explanation is good enough to explain whatever is going on. Even though you can’t always use Ockham’s Razor, it is very valuable.

It is Almost Always Helpful, Even if it Doesn’t Work: Ockham’s Razor is not meant to be an all-or-nothing principle. But even when you can’t make a full-blown choice of your beliefs based on Ockham’s Razor, it is still helpful. Roughly, this is because simple explanations require fewer contributing factors than complicated ones, and so in most cases simple explanations have much higher probabilities than complicated ones. Although Ockham’s Razor can be viewed as a way to choose between competing ideas that are equally good at explaining the world, it can be used in another way as well. Since simple explanations are favored over complicated ones, you can also apply Ockham’s Razor at the ‘beginning’. But if you apply it at the beginning, it isn’t conclusive. When investigating various ideas, each idea will have an “initial likelihood” in your mind – this is often called the prior probability. Ockham’s Razor has a place in evaluating these prior probabilities. For instance, the prior probability of Santa delivering presents on Christmas is much lower than the probability of everyone’s parents buying parents – but this is because, as adults, we know things that rule out Santa. But, suppose you are 3 or 4 years old. You don’t know enough about the world to decide whether Santa or your parents are better explanations of the presents underneath the tree. But, suppose your parents told you Santa brought the presents. Then since you have nothing else to go on, it is entirely reasonable to believe that Santa brings the presents, and so in this case the prior probability of Santa being the person who brings presents is quite high.

The thing to notice in this example is that the prior probability is always based upon what you already know. If you are 3 years old, the idea that Santa brings presents is the most simple, because all you have to do is believe your parents, whereas denying this requires devising why exactly your parents are lying to you – which is more complicated. But if you start off with the knowledge of an adult, then the entire situation flips on its head – because you know more information. To take this even further, if any of us were to actually live out a Christmas movie where Santa shows up, then probably the situation would change yet again to believing in Santa, because Santa’s actual existence is more straightforward than trying to explain how a bunch of reindeer were flying around. Although if you learn about special relativity theory, then the coin would probably flip again and the unlikelihood of flying reindeer might be balanced out by an outright violation of a law of physics.

Basically, Ockham’s Razor is a useful tool that states that simple ideas have an advantage over complicated ideas, so in order for a complicated idea to win it needs to gain an advantage somewhere else. There are plenty of other ways an idea can get an advantage. These will be discussed elsewhere.