When Phenomena Are Different

All of our ideas in physics require a certain amount of common sense in their application; they are not purely mathematical or abstract ideas.  We have to understand what we mean when we say that the phenomena are the same when we move the apparatus to a new position.  We mean that we move everything that we believe is relevant; if the phenomenon is not the same, we suggest that something relevant has not been moved, and we proceed to look for it.
—    Richard P. Feynman, Robert B. Leighton, and Matthew Sands
Quoted from: “The Feynman Lectures On Physics“
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