Logical Fallacies: Ockham's Razor

Pluralitas non est ponenda sine neccesitat.

“Do not multiply entities unnecessarily.”

Ockham's Razor is not a fallacy, but a means of avoiding some of the foolishness that besets critical thinkers. Though not an infallible guide, it is (to borrow a phrase from Sagan) "the means, in both science and religion, by which deep thoughts can be winnowed from deep nonsense.” 

The 14^th century divine, William of Ockham(usually spelled “Occam," but I prefer what I consider the older and more authentic spelling) was the originator of this logical principle. It isa rule of thumb, a guideline, not a hard-and-fast principle. It is oftenrestated as, “the simplest explanation which fits all the facts is probably the correct one.” (Emphasis added.) As someone elseonce said,  “When you hearhoofbeats, think horses, not unicorns.”

One of my favorite illustrations of this comes from the web site of Jose Wudka, professor of physics at UC Riverside:

Consider for example the following two theories aimed atdescribing the motion of the planets around the sun:

·      Theplanets move around the sun in ellipses because there is a force between any ofthem and the sun which decreases as the square of the distance.

·      Theplanets move around the sun in ellipses because there is a force between any ofthem and the sun which decreases as the square of the distance. This force isgenerated by the will of some powerful aliens.

Since the force between the planets and the sun determinesthe motion of the former and both theories posit the same type of force, thepredicted motion of the planets will be identical for both theories. The secondtheory, however, has additional baggage (the will of the aliens) which isunnecessary for the description of the system.

If one accepts the second theory solely on thebasis that it predicts correctly the motion of the planets one has also accepted the existence of aliens whosewill affect the behavior of things, despite the fact that the presence orabsence of such beings is irrelevant to planetary motion (the only relevantitem is the type of force). In this instance Ockham's Razor would unequivocallyreject the second theory. By rejecting this type of additional irrelevanthypotheses guards against the use of solid scientific results (such as theprediction of planetary motion) to justify unrelated statements (such as theexistence of the aliens) which may have dramatic consequences. In this case theconsequence is that the way planets move, the reason we fall to the ground whenwe trip, etc. is due to some powerful alien intellect, that this intellectpermeates our whole solar system, it is with us even now...and from here aninfinite number of paranoid derivations.

For all we know the solar system is permeated by an alienintellect, but the motion of the planets, which can be explained by the simpleidea that there is a force between them and the sun, provides no evidence ofthe aliens' presence nor proves their absence.

A more straightforward application of the Razor iswhen we are face with two theories which have the same predictions and theavailable data cannot distinguish between them. In this case the Razor directsus to study in depth the simplest of the theories. It does not guarantee that the simplest theory will be correct,it merely establishes priorities.
 
A related rule, which can be used to slice open conspiracytheories, is Hanlon's Razor: "Never attribute to malice that which can beadequately explained by stupidity."

http://phyun5.ucr.edu/~wudka/Physics7/Notes_www/node10.html