Simplifications are very useful and perfectly permissible for helping
us to grasp the essentials of the situation. We should, however,
always try to be clear about what simplifications we have made. In
this way, we know how to react to the inevitable discrepancies that
are found when we compare the results of our calculations with
measurements of the real things. When the discrepancies are of the
sort to be expected as a result of our simplifications, they confirm
that we probably understand at least a part of what is going
on. Sometimes, however, the discrepancies are not explainable in terms
of our simplifications, which reminds us firmly to look more closely
at the system under study.
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