"The logic of modern Physics" by Percy Williams Bridgman,1927

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The concepts of physics should be flexible in order not to restrict the possibilities of future experience.
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To provide flexibility,concepts should be defined operationally ;
the concept is synonymous with the operations by which one determines whether the concept applies,or what the measure of something is.
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There is no absolute time ;
time is measured by reference to clocks of some sort.
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Such concepts as causality,identify,force,energy,temperature,
and light become useful and meaningful as they are defined by reference to physical phenomena and operations of measurement.
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