ALMOST all progress in physics has come from comparing the unfamiliar with the familiar. One could scarcely have a better example than the story of Newton's apple—for this comparison assimilated the motion of the moon to the motion of a falling body. The great essential is to use an idea with which one is already familiar. It is less important that the idea should be really understood—if, indeed, anything in the physical world ever can be understood in any absolute sense. The falling of the apple was not understood then (one may perhaps doubt if we understand it much better now, when we say that it is a consequence of the curvature of space), but it was at least familiar, and the mind needs familiar images with which to work.
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Historical Studies in the Physical Sciences (1981)
Die Naturwissenschaften (1932)