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 Occam’s Razor.  It is a basic scientific principle. And it says, all things being equal, the simplest explanation tends to be the right one.

— Ellie Arroway, in the 1997 film Contact

The line of dialogue above is actually uttered twice by Jodie Foster in Contact, a film that popularized the concept of Occam’s Razor, a simple but elegant axiom which forms the bedrock on which all logic, investigation and scientific principle rests.  As a consummate fact-checker I have long held this fundamental parsimony in high regard.  Carl Sagan (who authored Contact) referenced the axiom again in his Demon Haunted World:  “Occam’s Razor.  This convenient rule-of-thumb urges us when faced with two hypotheses that explain the data equally well to choose the simpler.”

But these references above are really not so much a quote from Ockham as they are a very faint suggestion of what he actually wrote.  Simply, you will not find Occam’s Razor anywhere in Ockham’s writings.  William of Ockham, c. 1287–1347, was an English Franciscan friar and theologian, and to consider him any father of modern scientific thought doesn’t quite match up with reality.  The closest he ever came to the above quotes in any extant writing is “Numquam ponenda est pluralitas sine necessitate,” or “Plurality must never be posited without necessity.”  His words, arising out of a theological work, spent the next few centuries morphing and falling out of context.

The axiom reemerged in 1510 thanks to Ockham enthusiast Alessandro Achillini, as “quia multipicantur entia sine necessitate,” and a century after that by John Punch as “Non sunt multiplicanda entia sine necessitate,” or “Entities [things] must not be multiplied beyond necessity.”  A generation after Punch, in 1672, Isaac Newton seemed familiar with this revised phrase as he wrote, “I see no reason why they, that adhere to any of those hypotheses, should seek for other Causes of these Effects, unless (to use the Objectors argument) they will multiply entities without necessity.”

And so the axiom has evolved over the past seven centuries; translated, twisted and reinterpreted so many times that now the quote attributed to Ockham (even, really, the original meaning) bears only slight semblance to the words written by him.  In fact, Aristotle and Ptolemy wrote lines that are far more similar, more than a thousand years before.

  • Aristotle: “We may assume the superiority ceteris paribus [other things being equal] of the demonstration which derives from fewer postulates or hypotheses.”
  • Ptolemy:  “We consider it a good principle to explain the phenomena by the simplest hypothesis possible.”

Reality check: the axiom wasn’t even referred to as “Ockham’s Razor” until 1852!  Occam’s Razor may be a timeless truism, but the truth is, he never said it.

 ‘Occam’s Razor’ is a modern myth. There is nothing mediaeval in it, except the general sense of the post-mediaeval formula: Entia non sunt multiplicanda praeter necessitatem. This myth has come to full maturity and secured general assent, within the lifetime of many philosophers of the present day…

William Thorburn, 1918

 The widespread layperson’s formulation that ‘the simplest explanation is usually the correct one’ appears to have been derived from Occam’s razor.

Wikipedia (Emphasis added)