At the risk of wandering in and appearing ignorant in two newsgroups at once...
I think my answer to Karthik's original question would be that a sine wave only appears "perfect" because sines (and cosines) are (normally) the "units" of our analysis of periodic waveforms. Once you start down that path, that is, once you say that every waveform is "made up of" ("will be described as the combination of") some set of scaled and shifted sine waves, then your "units" will appear to be... um, "unitary".
But suppose some truly evil and sadistic mathematician decided that his classes would forever analyze periodic waveforms using some other basis, say square waves?
Suddenly a sine wave would be seen as a truly horrible combination of shifted and scaled square waves, a thing chock-full of "harmonics"... it would no longer be the pristine, pure thing it appeared when we looked at it through Fourier's eyes.
Does this make sense (even if my choice of square waves turns out to be poor)? Or did I miss something?
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