Occam's Razor (Law of Parsimony)
As found in William of Occam's (or Ockham) works:
"Pluralitas non est ponenda sine necessitate"
"Frustra fit per plura quod potest fieri per pauciora"
Translated into English as:
"Plurality should not be posited without necessity"
"Entities should not be multiplied unnecessarily"
or interpreted as
"one should not increase, beyond what is necessary, the number of entities required to explain anything"
A "useful statement of the principle for scientists" is:
"when you have two competing theories which make exactly the same predictions, the one that is simpler is the better."
Phil Gibbs states that:
Occam's razor is often cited in stronger forms than Occam intended, as in the following statements...
"If you have two theories which both explain the observed facts then you should use the simplest until more evidence comes along"
"The simplest explanation for some phenomenon is more likely to be accurate than more complicated explanations."
"If you have two equally likely solutions to a problem, pick the simplest."
"The explanation requiring the fewest assumptions is most likely to be correct."
... or in the only form which takes its own advice...
"Keep things simple!"
A useful saying from Einstein:
"Everything should be made as simple as possible, but not simpler."
Links to information about Occam's Razor:
What is Occam's Razor? by Phil Gibbs and Sugihara Hiroshi
Occam's Razor from 2think.org
Occam's Razor by F. Heyligen at Principia Cybernetica Web