On the Allegories of the Sacred Laws (1.11-1.15)

Again, the irrational portion of the soul is divisible into seven portions; - the five senses, and the organ of speech, and the power of generation.

Again, the motions of the body are seven; the six organic motions, and the rotatory motion.

Again, the secretions are seven—tears, mucus from the nose, saliva, the generative fluid, the two excremental discharges, and the sweat that proceeds from every part of the body. Moreover, in diseases the seventh day is the most critical period—and in women the catamenial purifications extend to the seventh day.

And the power of this number has extended also to the most useful of the arts—namely, to grammar. At all events, in grammar, the most excellent of the elements, and those which have the most powers, are the seven vowels. And likewise in music, the lyre with seven strings is nearly the best of all instruments; because the euharmonic principle which is the most dignified of all the principles of melody, is especially perceived in connection with it. Again, it happens that the tones of the voice are seven—the acute, the grave, the contracted, the aspirate, the lene, the long and the short sound.

The number seven is also the first number which is compounded of the perfect number, that is to say of six, and of the unit. And in some sense the numbers which are below ten are either generated by, or do themselves generate those numbers which are below ten, and the number ten itself. But the number seven neither generates any of the numbers below ten, nor is it generated by any of them. On which account the Pythagoreans compare this number to the Goddess always a virgin who was born without a mother,[*] because it was not generated by any other, and will not generate any other.