May 31, 2005

Science, religion, and Ockham's razor

A few days ago I was working in my backyard when I noticed that the outdoor thermometer that I had fixed to a fence had disappeared. The mountings were still there but had been pulled away slightly. I thought that maybe the wind had blown it off and so I looked at the ground underneath but the thermometer was not there. There is a bed of pachysandra nearby and I looked nearby in it but no luck. I was baffled.

I pondered the various options for explaining the missing thermometer. One was that the wind had been strong enough to rip the thermometer from its mounting and blow it farther away into the pachysandra. The other was that it had fallen to the ground below and had then been taken away by squirrels or the neighbor's cat. The third was that neighborhood children had borrowed it without permission for some experiment. The fourth was that the International Outdoor Thermometer Cartel (IOTC) had raised the price of these thermometers to such a high value that organized crime gangs were stealing them and selling them on the black market. The fifth option was that aliens had taken it away as a souvenir of their clandestine visit to Earth.

Given these options, I decided that #1 was the most likely one and looked in the pachysandra over a larger area, and sure enough. I found it.

The reason for this anecdote is that it illustrates that I used something that we all use all the time (whether we are consciously aware of it or not), and that is Ockham's razor to make choices among competing theories.

According to the Encyclopedia Brittanica, the principle behind Ockham's razor (also called the law of economy or the law of parsimony) was stated by the scholastic William of Ockham (1285–1347/49), as "Plurality should not be posited without necessity." The principle is also expressed as "Entities are not to be multiplied beyond necessity." Ockham did not himself use the word 'razor', that was added to his name later by others.

The principle gives precedence to simplicity, but there are two ways it can be used. In the first case (which is more closely aligned with Ockham's intent), it says that you should not postulate more elements for anything other than the minimum required. For example, in the case of my missing thermometer, if I postulated one theory that a cat had taken it and a competing theory was that a cat that had a striped tail and a scar on its forehead had taken it, then in the absence of any extra information, the former theory is to be preferred. The latter theory just adds elements that do not add any necessary information to the explanation. The application of this version of the principle is fairly straightforward. One seeks the smallest subset of elements of a theory that provides an adequate explanation of whatever you are trying to explain.

The more problematic (and common) use of Ockham's razor is when you try and apply it to a situation where there are two competing theories that share no common elements or there exist at least some necessary elements of one theory that the other does not possess. We commonly interpret Ockham's razor in those situations as requiring us to choose the simpler of the two theories. But simplicity may well lie in the eye of the beholder and it may not be easy to get agreement.

So, for example, in the case of the thermometer that was found some distance away from its mountings, the simpler explanation (for me at least) was that of the wind. If called upon, I could call upon Bernoulli's Principle and the laws of motion to support my preference. That explanation is simple enough to satisfy me.

But this may not be true for someone else. For them, a theory that alien vandals landed in my garden, tore the thermometer from its moorings, threw it away in the pachysandra and left in their spaceship, might be the "simpler" explanation in the eyes of someone who is a believer in the existence UFOs and space aliens. After all, it does not involve the use of calculus.

That is exactly the problem in many of the science and religion discussions, and we will see that in the next posting.


In a comment to a previous post, Amanda (a former student who graduated a few years ago and is now doing her PhD in astronomy) sent me a link to an excellent New Yorker article that goes straight to the core intelligent design argument, cutting through all the confusion that often surrounds such discussions. The article is well written and lays out the basic premises of ID as well as clears up some popular confusion about how evolution and natural selection work. I strongly recommend the article and gratefully thank Amanda for bringing it to my attention.


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Is this often-quoted caveat an example of Ockham's razor: "If you hear hoofbeats, you should think first of horses, not zebras." In a way it does seem like a similar means of solving a mystery. What do you think? And where did the horses/zebras example come from, do you know?

Thanks for the link to the New Yorker article. I've been trying to find the hard copy at the library, but so far no luck. I've heard it's excellent. Cathie

Posted by catherine on May 31, 2005 11:59 AM

I have no idea where the horse/zebra example comes from, not having heard of it before today.

The example could be used as one of the second forms of Ockham's razor.

Posted by Mano Singham on May 31, 2005 01:26 PM

Ah yes, Ockham's razor. Simplicity...
Ah yes, The Pull of gravitation, the pull we need of Dark Matter to hold the universal expansion in check... and, now the repulsion required of Dark Energy to work against the pull of gravitation to explain the now known accelerating universal expansion at the fringes of the universe...
Would you say a fair definition of a push is whenever pressure is brought to bear? Can you find any known pull accomplished without there being pressure brought to bear? Refer to the intimate point of contact required to pull anything, and at this frame of reference, what is happening?
Ah yes, maybe not so simple, a pull!?
Ah yes, the simplicity of a push...

Posted by Ron Davis on May 31, 2005 09:58 PM

I had been arguing with my close friend on this issue for quite a while, base on your ideas prove that I am right, let me show him your webpage then I am sure it must make him buy me a drink, lol, thanks.

- Kris

Posted by mietwagen spanien on October 17, 2010 10:47 AM

Excellent article.

I particularly enjoyed how you pointed out that simplicity is in the eye of the beholder.

That is the problem with terms like simplicity. They truly are subjective / relative. Depends on the task and person doing it.

~Gar from Build Trebuchets

Posted by Gar on January 10, 2011 06:54 PM

Maybe the aliens had taken it away. The thermometer was so bright that it attracted their attention.

Posted by drm on May 11, 2011 09:12 PM