1.6: What is Occam's Razor?
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This article is from the Scientific Skepticism FAQ, by Paul Johnson Paul@treetop.demon.co.uk with numerous contributions by others.
1.6: What is Occam's Razor?
Ockham's Razor ("Occam" is a Latinised variant) is the principle
proposed by William of Ockham in the fifteenth century that
"Pluralitas non est ponenda sine neccesitate", which translates as
"entities should not be multiplied unnecessarily". Various other
rephrasings have been incorrectly attributed to him. In more modern
terms, if you have two theories which both explain the observed facts
then you should use the simplest until more evidence comes along. See
W.M. Thorburn, "The Myth of Occam's Razor," "Mind" 27:345-353 (1918)
for a detailed study of what Ockham actually wrote and what others
wrote after him.
The reason behind the razor is that for any given set of facts there
are an infinite number of theories that could explain them. For
instance, if you have a graph with four points in a line then the
simplest theory that explains them is a linear relationship, but you
can draw an infinite number of different curves that all pass through
the four points. There is no evidence that the straight line is the
right one, but it is the simplest possible solution. So you might as
well use it until someone comes along with a point off the straight
line.
Also, if you have a few thousand points on the line and someone
suggests that there is a point that is off the line, it's a pretty
fair bet that they are wrong.
The following argument against Occam's Razor is sometime proposed:
This simple hypothesis was shown to be false; the truth was more
complicated. So Occam's Razor doesn't work.
This is a strawman argument. The Razor doesn't tell us anything about
the truth or otherwise of a hypothesis, but rather it tells us which
one to test first. The simpler the hypothesis, the easier it is to
shoot down.
A related rule, which can be used to slice open conspiracy theories,
is Hanlon's Razor: "Never attribute to malice that which can be
adequately explained by stupidity". This definition comes from "The
Jargon File" (edited by Eric Raymond), but one poster attributes it to
Robert Heinlein, in a 1941 story called "Logic of Empire".
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