Philo Judaeus De Opificio Mundi 126

And the power of this number does not exist only in the instances already mentioned, but it also pervades the most excellent of the sciences, the knowledge of grammar and music. For the lyre with seven strings, bearing a proportion to the assemblage of the seven planets, perfects its admirable harmonies, being almost the chief of all instruments which are conversant about music. And of the elements of grammar, those which are properly called vowels are, correctly speaking, seven in number, since they can be sounded by themselves, and when they are combined with other letters, they make complete sounds; for they fill up the deficiency existing in semi-vowels, making the sounds whole; and they change and alter the natures of the mutes inspiring them with their own power, in order that what has no sound may become endowed with sound.