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Occam's Razor: What Is It, Exactly?

Posted by Josh Rothman  May 18, 2011 08:00 AM

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Over at Rationally Speaking -- the blog of CUNY philosophy professor Massimo Pigliucci -- there's a fascinating debate unfolding about the meaning of Occam's razor. Occam's razor is an oft-cited principle which holds that simpler explanations, all things being equal, are usually better than more complex ones. Nowadays it seems that everyone is wielding the razor, from physicists and biologists to detectives and policy-makers -- and yet, Pigliucci argues, Occam's razor is actually a very strange idea, and probably a less useful one than many people think.


The Razor-wielding William of Occam.

Pigliucci draws on the philosopher Elliot Sober, who in a 1994 essay ("Let's Razor Occam's Razor") asked, in Pigliucci's words, "Why? On what basis are we justified to think that, as a matter of general practice, the simplest hypothesis is the most likely one to be true?" One possibility, of course, is that the simplest explanations tend to be true because the universe is, in some sense, simple. That's an ancient idea: Thomas Aquinas, for example, argued that, "if a thing can be done adequately by means of one, it is superfluous to do it by means of several, for we observe that nature does not employ two instruments where one suffices"; William of Occam, who invented the razor, did so in the 14th century. In fact, though, the universe isn't really simple in quite this way -- except, perhaps, in some deep mathematical sense. "The history of science," Pagliucci writes, "is replete with examples of simpler ('more elegant,' if you are aesthetically inclined) hypotheses that had to yield to more clumsy and complicated ones."

If Occam's razor isn't ontologically justified, Pigliucci continues, then it must be epistemically useful: favoring simple answers must make it easier, over time, to find the right answer. This is probably true, he concludes, in a variety of fields -- but not because of some mysterious, underlying fact about knowledge that cuts across disciplines. The reasons why Occam's razor is useful in physics are probably different from the reasons it's useful in biology. In most cases, moreover, you can get away with favoring simpler explanations only because there's a huge quantity of (quite complicated) background knowledge hiding inside the simplicity. It's actually because of all that background knowledge that we can present ideas simply in the first place! "Occam's razor," Pigliucci concludes, "is a sharp but not universal tool, and needs to be wielded with proper care." It's limited: unfortunately, for instance, "one cannot eliminate flying saucers a priori just because they are an explanation less likely to be the correct than, say, a meteor passing by."

That's not to say, obviously, that Occam's razor isn't tremendously useful. One commenter points to this fascinating paper in the Handbook of the Philosophy of Science, written by philosopher Kevin Kelly. The point of the razor, Kelly argues, isn't that it helps you find out the truth: it's that, as a tool for thinking, it gets you to the truth faster. Kelly writes:

Occam's razor does not point at the truth, even with high probability, but it does help one arrive at the truth with uniquely optimal efficiency, where efficiency is measured in terms of such epistemically pertinent considerations as the total number of errors and retractions of prior opinions incurred before converging to the truth and the elapsed times by which the retractions occur. Thus, in a definite sense, Ockham’s razor is demonstrably the uniquely most truth-conducive method for inferring general theories from particular facts -- even though no possible method can be guaranteed to point toward the truth with high probability in the short run.

The moral of the story: you cannot cite "Occam's razor" non-problematically in arguments with your friends. It's really useful only within lengthy, sustained processes of inquiry.

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About brainiac Brainiac is the daily blog of the Globe's Sunday Ideas section, covering news and delights from the worlds of art, science, literature, history, design, and more. You can follow us on Twitter @GlobeIdeas.
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