Plutarch De animae procreatione in Timaeo 13

[13] To proceed by way of multiplication,—twice 3 make 6, and 4 times 9 thirty-six, and 8 times 27 produce 216. Thus six appears to be a perfect number, as being equal in its parts; and it is called matrimony, by reason of the mixture of the first even and odd. Moreover it is composed of the original number, which is one, of the first even number, which is two, and the first odd number, which is three. Then for 36, it is the first number which is as well quadrangular as triangular, being quadrangular from 6, and triangular from 8. ¹ The same number arises from the multiplication of the first two square numbers, 4 and 9; as also from the addition of the three cubical numbers, 1, 8, and 27, which being put together make up 36. Lastly, you have a parallelogram with unequal sides, by the multiplication of 12 by 3, or 9 by 4. Take then the numbers of the sides of all these figures, the 6 of the square, the 8 of the triangle, the 9 for the one parallelogram, and the 12 for the other; and there you will find the proportions of all the concords. For 12 to 9 will be a fourth, as nete to paramese. To eight it will prove a fifth, as nete to mese. To six it will be an octave, as nete to hypate. And the two hundred and sixteen is the cubical number proceeding from six which is its root, and so equal to its own perimeter.