De institutione musica II.14: Cur ita appellatae sint digestae superius medietates.
One of them is called the arithmetic mean because between the terms the difference is equal according to number. The second is called geometric because the quality of the proportion is similar. The harmonic is so called because it is so fitted that in the differences and terms an equality of proportions is considered. Of these a more careful discussion has been given in the Arithmetic; now we run through them only to recall them.