Principle Of Parsimony

What science does, in fact, is to select the simplest formula that will fit the facts. But this, quite obviously, is merely a methodological precept, not a law of nature. If the simplest formula ceases, after a time, to be applicable, the simplest formula that remains applicable is selected.

-- Russell, B. (1953). On the notion of cause, with applications to the free-will problem. In H. Feigl & M. Brodbeck (Eds.), Readings in the philosophy of science (pp. 387-407). New York: Appleton-Century-Crofts.
In other words, UseTheSimplestFormulaThatFitsTheFacts?, DoTheSimplestThingThatWorks?, UseTheSimplestMetricThatPredicts?, etc.
The pain arises, of course, during that period between the time the simplest formula ceases to be applicable and the time the simplest remaining formula is applied. -- anonymous

And, sometimes the old, simpler formula is still useful. Dealing with "ordinary" scales of size, mass and time, Newton is still great stuff. I don't need to take relativity into account when I'm moving at a tiny fraction of C.

This is very much related to TheStructureOfScientificRevolutions.

On the other hand, YouArentGonnaNeedIt.
See also OccamsRazor, KeepItSimple, DoSimpleThings, DoTheSimplestThingThatCouldPossiblyWork, HeuristicRule

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