Ockham's razor

And the law of parsimony

David Dufty, April 2007

Ockham's razor was originally proposed by a monk named William of Ockham. He did not call it "Ockham's razor" or even "my razor." This is a name that has been given to it over time. Ockam's razor is the principle that if you have to choose between two explanations (or theories), choose the simplest one.

Another version of this principle is the Law of parsimony . This says that if you are choosing between two theories, choose the one with the fewest assumptions. Assumptions here means claims of fact that have no evidence.

A theory that doesn't have many assumptions, and is very simple, is called a parsimonious theory.

An example of a parsimonious theory is Einstein's equation: E=mc 2

This only has one assumed number: the number c (which is the speed of light). It is simple and elegant, which is one reason why it is such a great theory.

Most conspiracy theories fail the test of Ockham's razor.

For example, take the idea that 9/11 was a conspiracy. This involves the following:

An elaborate plan by the CIA (or someone): massive co-ordination and government funding, yet no patriotic people involved leaked the information or blew the whistle to the media.

These assumptions are actually clusters of assumptions. For example, the co-ordination assumes the following:
1) the president planned to kill Americans

2) people close to the president were in on the plan

3) a government agency funded the plan

4) no-one in the agency had the courage or the will to contact the media

5) the funding was somehow kept secret from congress

6) the accounting procedures used to hide the funding were sufficiently elaborate and clever as to evade detection- this would require high-level involvement of other government agencies, such as treasury

7) treasury was in on the plan

8) no-one in treasury thought to contact the media

Etc. So many assumptions: so many claims with no evidence!!!!

Yet the alternative account is the following:

1) A bunch of guys with a grudge against America decided to hijack planes and crash them

2) they implemented this plan.

Fewer assumptions, therefore more plausible.

In the lecture, I gave an example of a parsimonious scientific theory that accounted for a lot of data: the theory of evolution.

For example, let's look at phobias.

The top five phobias are:

Bugs, Mice, Snakes, and Bats

Heights

Water

Crowds

Storms

Yet the top five causes of accidental death are:

Motor vehicles

Falls

Poison

Drowning

Fires

why don't more people have a phobia of cars?

A simple explanation is that phobias are evolved cognitions. People that are scared of snakes are more likely to have children than people who think snakes are cuddly. Over time, the theory goes, each generation had more and more people who were susceptible to developing a fear of snakes.

The things that people are most likely to develop phobias of are things that have been killing people for as long as people have been around. Cars have barely been around for a hundred years- this is not long enough for the population to acquire a natural fear of them.

Another piece of data that has been explained in terms of evolution is the differing behavior between men and women in relationships.

Researchers at Hawaii university employed an attractive man and an attractive woman to approach people of the opposite sex and ask them on a date. The requests varied in explicitness. Men were more likely to say yes to an explicit request ("will you go to bed with me") whereas women never responded with yes to an explicit request, but were more likely to respond to a less explicit request ("will you have dinner with me").

The field of evolutionary psychology has an explanation for this: men who follow short-term goals are more likely to have more children than men who follow long-term goals.

The opposite is true of women: long-term relationships are in their interest, because a woman without a man will have a harder time raising a child to adulthood.

Thus, according to evolutionary psychologists, there has been evolutionary pressure for men and women to pursue slightly different strategies.

Evolution also explains the emergence of drug resistant organisms such as Golden Staph and Legionaires disease. Whenever a new vaccine or anti-biotic is developed it is very successful at destroying disease.

However, bacteria especially have a high rate of genetic information transmission and variance. It is only a matter of time before some bacteria survive. The bacteria that survive the medicine breed and grow, and soon the disease is drug-resistant.

These are three examples (of many) but the point is this: the same theory explains multiple effects. This reduces the overall number of explanations needed. If you only need one theory to explain lots of things, it makes everything simple.

Because the theory of evolution fits with Ockham's razor so well: it requires so few assumptions: it is an attractive theory to researchers looking for explanations of why things are the way they are.

© Copyright 2007 David Dufty