I don't see a good online reference, maybe I read this a guitar book somewhere, but essentially the tones for A, B, C, D, E, F, G notes were picked out by the ancient Greeks (who referred to them as Alpha, Beta, Gamma ... so on) In fact Pythagoras came up with a mathematical tuning system http://en.wikipedia.org/wiki/Pythagoras#Musical_theories_and...
The 7 natural notes are based on a 7-tone equal temperament (equal tonal spacing), which gives you enough equal spacing between notes to make nice harmonies (nice complimentary resonance) so this sort of a 7-tone minimal palette, and 5 "accidentals" fill the spaces in between those 7 tones to make a fuller 12-tone equal temperament palette.
The 7 natural notes are based on a 7-tone equal temperament (equal tonal spacing), which gives you enough equal spacing between notes to make nice harmonies
But they are not equally spaced. The tonal gap between A and B is twice as big as the gap between B and C, for example (look at the frequencies on a logarithmic scale).
The parent is being slightly confusing by bringing temperament into the discussion. They mean that the 12 notes are evenly spaced on an evenly tempered instrument, which means of course that you don't get the 'perfect' fourth or fifth you would get on a naturally tempered instrument (the frequencies are very slightly off).
Nearly all music we hear now is evenly tempered, but that is a pretty recent innovation.
The 7 natural notes are based on a 7-tone equal temperament (equal tonal spacing), which gives you enough equal spacing between notes to make nice harmonies (nice complimentary resonance) so this sort of a 7-tone minimal palette, and 5 "accidentals" fill the spaces in between those 7 tones to make a fuller 12-tone equal temperament palette.