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Occam's Razor: A Useful Tool in Logic

The term "Occam's Razor" comes from a misspelling of the name William of Ockham. Ockham was a brilliant theologian, philosopher, and logician in the medieval period. One of his rules of thumb has become a standard guideline for thinking through issues logically. Occam's Razor is the principle that, "non sunt multiplicanda entia praeter necessitatem" [i.e., "don't multiply the agents in a theory beyond what's necessary."] What does that mean? If two competing theories explain a single phenomenon, and they both generally reach the same conclusion, and they are both equally persuasive and convincing, and they both explain the problem or situation satisfactorily, the logician should always pick the less complex one. The one with the fewer number of moving parts, so to speak, is most likely to be correct. The idea is always to cut out extra unnecessary bits, hence the name "razor." An example will help illustrate this.

Suppose you come home and discover that your dog has escaped from the kennel and chewed large chunks out of the couch. Two possible theories occur to you. (1) Theory number one is that you forgot to latch the kennel door, and the dog pressed against it and opened it, and then the dog was free to run around the inside of the house. This explanation requires two entities (you and the dog) and two actions (you forgetting to lock the kennel door and the dog pressing against the door). (2) Theory number two is that some unknown person skilled at picking locks managed to disable the front door, then came inside the house, set the dog free from the kennel, then snuck out again covering up any sign of his presence, and then relocked the front-door, leaving the dog free inside to run amok in the house. This theory requires three entities (you, the dog, and the lockpicking intruder) and several actions (picking the lock, entering, releasing the dog, hiding evidence, relocking the front door). It also requires us to come up with a plausible motivation for the intruder--a motivation that is absent at this point.

Either theory would be an adequate and plausible explanation. Both explain the same phenomenon (the escaped dog) and both employ the same theory of how, i.e., that the latch was opened somehow, as opposed to some far-fetched theory about canine teleportation or something crazy like that.

Which theory is most likely correct? If you don't find evidence like strange fingerprints or human footprints or missing possessions to support theory #2, William of Ockham would say that the simpler solution (#1) is most likely to be correct in this case. The first solution only involves two parts--two entities and two actions. On the other hand, the second theory requires at least five parts--you, the dog, a hypothetical unknown intruder, some plausible motivation, and various actions. It is needlessly complex. Occam's basic rule was "Thou shalt not multiply extra entities unnecessarily," or to phrase it in modern terms, "Don't speculate about extra hypothetical components if you can find an explanation that is equally plausible without them." All things being equal, the simpler theory is more likely to be correct, rather than one that relies upon many hypothetical additions to the evidence already collected.



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Copyright Dr. L. Kip Wheeler 1998-2018. Permission is granted for non-profit, educational, and student reproduction. Last updated April 24, 2018. Contact: kwheeler@cn.edu Please e-mail corrections, suggestions, or comments to help me improve this site. Click here for credits, thanks, and additional copyright information.