Philo: On the Creation

III. (13) And he says that the world was made in six days, not because the Creator stood in need of a length of time (for it is natural that God should do everything at once, not merely by uttering a command, but by even thinking of it); but because the things created required arrangement; and number is akin to arrangement; and, of all numbers, six is, by the laws of nature, the most productive: for of all the numbers, from the unit upwards, it is the first perfect one, being made equal to its parts, and being made complete by them; the number three being half of it, and the number two a third of it, and the unit a sixth of it, and, so to say, it is formed so as to be both male and female, and is made up of the power of both natures; for in existing things the odd number is the male, and the even number is the female; accordingly, of odd numbers the first is the number three, and of even numbers the first is two, and the two numbers multiplied together make six.

(14) It was fitting therefore, that the world, being the most perfect of created things, should be made according to the perfect number, namely, six: and, as it was to have in it the causes of both, which arise from combination, that it should be formed according to a mixed number, the first combination of odd and even numbers, since it was to embrace the character both of the male who sows the seed, and of the female who receives it. (15) And he allotted each of the six days to one of the portions of the whole, taking out the first day, which he does not even call the first day, that it may not be numbered with the others, but entitling it one, he names it rightly, perceiving in it, and ascribing to it the nature and appellation of the limit.