(106) And it is also affirmed for the particular praise of the number seven, that it has a very admirable rank in nature, because it is composed of three and four. And if any one doubles the third number after the unit, he will find a square; and if he doubles the fourth number, he will find a cube. And if he doubles the seventh from both, he will both a cube and a square; therefore, the third number from the unit is a square in a double ratio. And the fourth number, eight, is a cube. And the seventh number, being sixty-four, is both a cube and a square at the same time; so that the seventh number is really a perfecting one, signifying both equalities, ùthe plane superficies by the square, according to the connection with the number three, and the solid by the cube according to its relationship to the number four; and of the numbers three and four, are composed the number seven.

XXXVII. (107) But this number is not only a perfecter of things, but it is also, so to say, the most harmonious of numbers; and in a manner the source of that most beautiful diagram which describes all the harmonies, that of fourths, and that of fifths, and the diapason. It also comprises all the proportions, the arithmetical, the geometrical, and moreover the harmonic proportion. And the square consists of these numbers, six, eight, nine, and twelve; and eight bears to six the ratio of being one third greater, which is the diatessaron of harmony. And nine bears to six the ratio of being half as great again, which is the ratio of fifths. And twelve is to six, in a twofold proportion; and this is the same as the diapason. (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. (109) But there are two ways of judging of harmonic proportion. One when, whatever ratio the last number bears to the first, the excess by which the last number exceeds the middle one is the same as the excess by which the middle number exceeds the first. And any one may derive a most evident proof of this from the numbers before mentioned, six, and eight, and twelve: for the last number is double the first. And again, the excess of twelve over eight is double the excess of eight over six. For the number twelve exceeds eight by four, and eight exceeds six by two; and four is the double of two. (110) And another test of harmonic proportion is, when the middle term exceeds and is exceeded by those on each side of it, by an equal portion; for eight being the middle term, exceeds the first term by a third part; for if six be subtracted from it, the remainder two is one third of the original number six: and it is exceeded by the last term in an equal proportion; for if eight be taken from twelve, the remainder four is one third of the whole number twelve.