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Now in mathematics it is legitimate to seek transformations through which certain quantities (such as the physical laws of nature and the velocity of light in relativity theory) remain invariant. In fact, the action of various physical devices which “recognize” or respond identically to certain simple objects can be treated in terms of such transformations. Thus the essential part of physical “recognizing” instruments is usually a filter—whether it be a mechanical sieve, an optical filter, or a tuned electrical circuit —which “passes” only quantities of the kind it is required to identify and rejects all others. Mathematically, the situation here is that, in a perfect filter, the transformation leaves the desired quantity unaltered, but reduces all others to zero.