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"Strictly speaking, Newton's laws are epistemologically circular, so in practice we must apply it both inductively and deductively. First we use them inductively with our primitive observations to identify inertial coordinate systems by observing how things behave. Then at some point when we've gained confidence in the inertialness of our coordinates, we begin to apply the laws deductively, i.e., we begin to deduce how things will behave with respect to our inertial coordinates. Ultimately this is how all physical theories are applied, first inductively as an organizing principle for our observations, and then deductively as "laws" to make predictions."

From here: http://mathpages.com/rr/s3-01/3-01.htm

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