Occam's razor (also known as lex parsimoniae), an often cited principle of parsimony, economy, or succinctness used in problem-solving. It has also been expressed as the KISS principle or "Keep it simple stupid!".
It states that among competing hypotheses, the one with the fewest assumptions should be selected. Although other, more complicated solutions may ultimately prove correct, in the absence of certainty the fewer assumptions made, the better.
Who does not remember university, where loads of students were (and constantly are) told "When you hear hoofbeats, look for horses, not zebras," which means look for the simplest, most common explanation first.
A more useful approach may be to think of the probability of the diagnosis in general, with decreasing probability down the list:
- typical manifestation of a common disorder
- atypical manifestation of a common disorder
- typical manifestation of a rare disorder
- atypical manifestation of a rare disorder
Often, suggesting rare diseases will rarely prove right.
History and etymology
William of Occam (1285-1348) was a member of the Franciscan order of friars, who hailed from the village of Occam (or Ockham) in the county of Surrey in England. He is remembered as an important philosopher of the medieval period. The fundamental belief that the simplest explanations should be sought for phenomena harks from the Classical Greek period, but William of Occam brought it to the attention of many more through his writings. This was expressed by him in Latin as "entia non sunt multiplicanda praeter necessitate" or "entities must not be multiplied beyond necessity". It is important to note that he was not thinking about the diagnostic method in medicine when he wrote his razor.
The use of the word razor in this context sounds odd to the modern ear, but it is used in philosophy to describe an underlying proposition that permits the 'shaving off' of superfluous explanations!
'Lex parsimoniae' is the Latin for law of parsimony.