About 'first principles'

I don' t understand why we called the process of finding a derivative from the definition 'finding the derivative from first principles'.
Why we have 's' after "principle"?

Quote:

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Originally Posted by ling_c_0202

I don' t understand why we called the process of finding a derivative from the definition 'finding the derivative from first principles'.
Why we have 's' after "principle"?

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Because there is not a single principle that we start from but a number of
them.

RonL