The term “Occam’s Razor” comes from a misspelling of the name William of Ockham. William Ockham (circa 1287–1347) was an English Franciscan friar, 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 [in Latin], “non sunt multiplicanda entia praeter necessitatem” [in English “don’t multiply the agents in a theory beyond what’s necessary.”]
Said most simply: “the simplest solution is almost always the best.”
What does that mean? If two competing theories explain a single phenomenon, and they both generally reach a similar 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 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.
- 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). Or…
- 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 what needs to be explained, which is the escaped dog, and both use 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 additional 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.
A variation used in medicine is called the “Zebra“: a physician should reject an exotic medical diagnosis when a more commonplace explanation is more likely, derived from Theodore Woodward‘s dictum “When you hear hoofbeats, think of horses, not zebras”.
As explained by John G. Sotos, medical novices are predisposed to make rare diagnoses because of (a) the availability heuristic (“events more easily remembered are judged more probable”) and (b) the phenomenon first enunciated in Rhetorica ad Herennium (circa 85 BC), “the striking and the novel stay longer in the mind.” Thus, the aphorism is an important caution against these biases when teaching medical students to weigh medical evidence.
Medical Diagnosticians have warned, however, that “zebra”-type diagnoses must nonetheless be held in mind until the evidence conclusively rules them out.
Occam’s Razor is a tool we all use knowingly or unknowingly. If your personality is high in trait neuroticism then you need to be especially aware of this bias.
Two very easy questions to help quickly guide you to a better explanation for your situation:
- Is there evidence that exists that my explanation does NOT account for? (If so you’ve used Occam’s Razor to cut too much off)
- Does my explanation require evidence that does not exist? Is there a more simple explanation that the existing evidence supports? (If so use Occam’s Razor to cut your explanation down to fit the evidence).