This week's definition of Occam's Razor is that the simplest explanation is usually the correct one. As I've been taught, though, that's actually an oversimplification of the rule - and one that actually renders it useless, since 10 people will give you 11 opinions about what the simplest explanation is.

I was taught the rule is "The explanation that requires the fewest assumptions of facts-not-in-evidence is usually the correct one."

The term Occam's razor was coined in the middle of the 19th century by William Hamilton, a Scottish mathematician: Different versions exist:

• Entia non sunt multiplicanda praeter necessitatem Entities should not be multiplied unnecessarily. (Attributed to Occam, does not occur in any of his known works.)
• Numquam ponenda est pluralitas sine necessitate Plurality must never be posited without necessity.
• Frustra fit per plura quod potest fieri per pauciora It is futile to do with more things that which can be done with fewer.

For more information see William M Thorburn (1918) "The Myth of Occam's Razor" in Mind 27, pp.345–353 (link). Also, note that the philosopher in question was named William of Ockham (or Occam). Cf. Leonardo or Da Vinci.

When a medical expert in a diagnostic discussion invokes the name of Occam, it's fun to offer up the converse.... Hickam's dictum ... "A patient can have as many diseases as he damn well pleases".

Welcome, diamondon. Who is or was Hickam, please?

While searching out the supposed Einstein quote about simplicity I cam across this little rambling on the subject from Will.Whim.

RE: zebras, the Quagga was a subspecies of the Plains Zebra, occurring in a limited range in South Africa. It has been extinct since the early 1880's (source Wikipedia). I'm not sure how it applies to the example given, except perhaps in the context of hypochondria, i.e., the explanation being a nonexistent illness?

I'm not sure how it applies to the example given, except perhaps in the context of hypochondria, i.e., the explanation being a nonexistent illness? Hi, Robert, and welcome. Well--in the W.A.D. Anu stated: Medical students are told, for example, "When you hear hoofbeats, think horses, not zebras." Barring countries where one is actually likely to hear zebras, it essentially means look for the most common cause(s) of symptoms first, rather than immediately running through the literature on obscure diseases. Absent special circs, such as the patient having recently traveled to some exotic place.

I remember one episode of Untold Stories of the E.R. where a young woman came in complaining, iirc, of severe headaches and nausea. I'm not a medical person [shudder] so I won't try to list what was done first, second, etc. But the doc, on visit after visit, tried all the standard tests and pain relievers, etc., and could never find anything; finally they sent her to Psychiatric, which also didn't help. She decided to try leaving for college on scehedule anyway, and I guess for sentimental reasons--he felt guilty about not being able to help her--the doc decided to walk her to her car. It was something several decades old which someone had restored. He accepted her offer of a short ride, and he began to get a headache. They rushed back and got blood tests: it turned out that carbon monoxide was leaking into the interior of the car. Mystery solved.

I think Robert was referring to the addition of quaggas in the list of things not to think about.

And the inverse relationship between the frequency of the quaggas' presence and the experience of the clinician. I've seen quite a few zebras, but still waiting for a quagga.

Em. Okay. Since quaggas weren't mentioned in the W.A.D., I thought zebras was the main subject of his post. Oops.