[108] The number seven comprises also, as I have said, all the proportions of arithmetrical proportion, from the numbers six, and nine, and twelve; for as the number in the middle exceeds the first number by three, it is also exceeded by three by the last number. And geometrical proportion is according to these four numbers. For the same ratio that eight bears to six, that also does twelve bear to nine. And this is the ratio of thirds. Harmonic ratio consists of three numbers, six, and eight, and twelve.