Whorf and Sapir both drew explicitly on Albert Einstein's principle of general relativity; hence linguistic relativity refers to the concept of grammatical and semantic categories of a specific language providing a frame of reference as a medium through which observations are made. Following an original observation by Boas, Sapir demonstrated that speakers of a given language perceive sounds that are acoustically different as the same, if the sound comes from the underlying phoneme and does not contribute to changes in semantic meaning. Furthermore, speakers of languages are attentive to sounds, particularly if the same two sounds come from different phonemes. Such differentiation is an example of how various observational frames of reference leads to different patterns of attention and perception.