Occam's razor

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This page in a nutshell: Occam's razor is a logical device intended to help a person select the most reasonable theoretical alternative. It states that a simple explanation is preferable to a complex one if it explains the data equally well. Occam's razor can only be used to decide between two hypotheses which both explain the data equally well. It cannot be used to favor an hypothesis because it is conventional.

Occam's razor states that we should accept the simplest full explanation of the data. For example, if we see that a pitcher has been knocked over, it is simpler to speculate that the dog did it than that a stranger sneaked in off the street and did it.

Contents 1 Use in skepticism 2 An example of the misuse of Occam's razor 3 See also 4 References

Occam's razor is not a logical principle, and it is not a physical principle like the conservation of energy. Nature is not bound by it, and thus nature is free to have complex structures, even though their explanation must be complex. For example a cell is very complex, and requires a complex explanation. But that does not mean that we should speculate that cells do not exist. Rather, it means that if we have two explanations which describe equally well how cells work, we should accept the simpler explanation.

There is another way of stating Occam's razor. "Entities should not be multiplied unnecessarily." This means that we should choose the hypothesis with the fewest factors, as long as it explains our observations equally well. Taking our example, we know that the dog is in the house, but we have to speculate about another entity if we hypothesize that a stranger sneaked in off the street to knock over the pitcher. In that case, we have multiplied (increased) the entities in our hypothesis. However, if the pitcher had been picked up and placed on the table instead of knocked over, we would have to hypothesize a more complex explanation than "the dog did it."

To take a "real-world" example, when applying Occam's razor we should not speculate that God created the universe unless we need to. The universe is known to be perfectly balanced for life. If this is the only universe in existence, then we would have to accept that a God created it, because that is a simpler explanation than speculating that the universe came into existence perfectly balanced for life by pure chance. But if there are many universes, then the universe had many random chances to be perfectly balanced for life. Thus, we do not have to create another entity (God) to explain our observations.

Use in skepticism

Occam's razor is often abused in skepticism, because it is combined with the principle "extraordinary claims require extraordinary evidence" in such a way as to allow a complex explanation to carry more weight. Instead of sticking to the simplest explanation of the data, skeptics multiply entities (factors in their hypothesis). The razor is also used as if it meant "the most conventional explanation is likely to be true." Take another example:

A pitcher of water has been spilled. The house where it is spilled is said to have a poltergeist. Many people have reported this. The house is guarded by an alarm system, which makes it impossible to enter from the outside without being seen. Everyone agrees the alarm is foolproof. However, because ghosts are unlikely, a skeptical explanation is that a prankster tunneled into the house from the road 50 feet away, then tipped over the pitcher and escaped, filling in the tunnel behind him. This explanation multiplies entities, but because it is deemed more likely that a human exists than a poltergeist (because a poltergeist is an extraordinary claim), it is considered the better explanation. Thus the unlikelihood of a poltergeist is weighed against the unlikelihood of the tunnel. To bolster the argument, Occam's razor is invoked by saying that we do not have to postulate an unknown "entity" (a poltergeist). In such an argument, only extraordinary phenomena are counted as "entities" which we "multiply," while conventional phenomena (pranksters) are not counted as "multiplied entities," even if they are extremely unlikely.

In this example it is true in that the skeptical explanation requires fewer entities to exist in the real world (we don't have to add the entity "poltergeist"). But Occam's razor is not a way to determine what exists in the real world. Nevertheless Occam's razor is often incorrectly used to decide which data you would be more likely to find if you could find the data. Those who use the Razor in this way do not care if factors are multiplied in an explanation, as long as all the factors are conventional. In our example, it does not matter to skeptics how many more factors the prankster explanation has or how unlikely they are, because this explanation has no extraordinary claims like "poltergeist."

To Occam's razor, an explanation which is abnormal and simple has equal status with an explanation which is normal and simple. Occam's razor is merely a way of deciding which of two equally explanatory and equally probable hypotheses to choose. Again, for Occam's razor normal and abnormal explanations have equal merit:

Normal and simple
equals
Abnormal and simple
According to Occam's razor.^[1]

Whether or not a claim is extraordinary has no relationship to whether or not it is simple. After all, the simplest explanation for a spilled pitcher is that it fell over by itself. This is the hypothesis favored by Occam's razor. It is improbable however because we have never before observed self-tipping pitchers.  The fact that the simplest explanation of why the pitcher tipped over is wrong shows why Occam's razor cannot be used to decide which hypothesis is right but only to decide which of two equally likely explanatory hypotheses is better (pending a test of the hypothesis). Occam's razor cannot decide between a poltergeist and a prankster.

Occam's razor should not be used to help us decide what is real or likely to be real. Such things are to be decided by evidence. It is an abuse of Occam's razor to do anything with it besides choose the simpler of two otherwise equal hypothesis.^[2]

In argument, Occam's razor is often used to defend baroque explanations with many unverified assumptions. In such cases "Occam's razor" stands in for the principle that "the most conventional explanation is likely to be true." Such conventional explanations are favored above simple but extraordinary explanations. This may or may not be a good way to do science, but it should be clearly stated for what it is, and Occam's razor should be kept sharp for when it is appropriate. There is no magical rule which relieves us of the need to collect and analyze data and to form our hypotheses based on what we deem the most probable reality. This is a messy process which is always filled with uncertainty.

Summary: Don't use Occam's razor in place of the maxim that the most conventional explanation is most likely to be true.

An example of the misuse of Occam's razor

According to the section titled Skeptical scientific investigation in the Wikipedia article on the "Paranormal,"

Scientific skeptics advocate critical investigation of claims of paranormal phenomena: applying the scientific method to reach a rational, scientific explanation of the phenomena to account for the paranormal claims, taking into account that alleged paranormal abilities and occurrences are sometimes hoaxes or misinterpretations of natural phenomena. A way of summarizing this method is by the application of Occam's razor, which suggests that the simplest solution is usually the correct one.^[3]

This is wrong in that Occam's razor tells us which hypothesis to pursue, not which is correct. The razor is a practical rule of thumb. It also assumes that a conventional explanation will always be simpler. In reality, however, it may be much more complex. This passage was obviously written by the skeptical cabal at Wikipedia. Even Wikipedia's own Occam's razor article states that:

Simpler theories are preferable other things being equal. The other things in question are the evidential support for the theory^[4] Therefore, according to the principle, a simpler but less correct theory should not be preferred over a more complex but more correct one.

See also

Vinstonas Wu on Occam's razor

References

1. ↑ Another way of saying this is that Occam's razor does not distinguish between likely and unlikely explanations. Likelihood or unlikelihood have to be established based on data, and data re not always simple.
2. ↑ "91. The plain consequence is (and it is a general maxim worthy of our attention), 'That no testimony is sufficient to establish a miracle,unless the testimony be of such a kind, that its falsehood would be more miraculous, than the fact, which it endeavours to establish; and even in that case there is a mutual destruction of arguments, and the superior only gives us an assurance suitable to that degree of force, which remains, after deducting the inferior.' When anyone tells me, that he saw a dead man restored to life, I immediately consider with myself,whether it be more probable, that this person should either deceive or be deceived, or that the fact, which he relates, should really have happened. I weigh the one miracle against the other; and according to the superiority, which I discover, I pronounce my decision, and always reject the greater miracle. If the falsehood of his testimony would be more miraculous, than the event which he relates; then, and not till then, can he pretend to command my belief or opinion." -An Enquiry Concerning Human Understanding by David Hume et al
3. ↑ Three skeptics' debate tools examined,Template:Dead link accessed July 1, 2007
4. ↑ "when you have two competing theories which make exactly the same predictions, the one that is simpler is the better."Usenet Phyics FAQs