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S16e. When is a theory preferred to another one?
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Frequently, Ockham's razor 
   ''frustra fit per plura quod potest fieri per pauciora'', 
that we should not use more degrees of freedom than are 
necessary to model a phenomenon, is invokes to argue that the theory
with the fewest parameters is the best. But this is true only
when taken with many grains of salt. 

Chemists prefer as a starting point of their deepest investigations
the theory based on Dirac-Fock theory or even cruder approximations, 
treating the nuclei (for large problems even atoms) as elementary. 
This gives them all the information they need, while they can deduce 
nothing at all from the standard model which is supposed to be a much 
more exact and general theory.

Thus what is preferred depends a lot on which use can be made of it
Ockham's razor is appropriate only if two theories allow the same
deductions with a similar amount of work, or if the more parsimonious
theory is even superior in allowing one to derive the desired 
properties. 

Nothing in science is against a complicated model if it gives more ready
access to the quantities of interest than a formally simpler but 
computationally more difficult or even untractable formulation.

Given only the standard model +classical relativity 
(allegedly correctly describing all phenomena of the world at 
accessible energies, distances, and accuracy), we'd know very little 
about our world, and only very inaccurately. Not even the masses of 
the nuclei can be predicted at present with any confidence, let alone 
the properties of water or gold.

And given only string theory (a theory without any free parameter), 
we'd know essentially nothing about our world.

(See http://rz70.rz.uni-karlsruhe.de/~ed01/Hyle/Hyle3/hoffman.htm
for further discussion of Ockham's razor.)