[60] They were also created to serve as measures of time; for it is by the appointed periodical revolutions of the sun and moon and other stars, that days and months and years are determined. And moreover it is owing to them that the most useful of all things, the nature of number exists, time having displayed it; for from one day comes the limit, and from two the number two, and from three, three, and from the notion of a month is derived the number thirty, and from a year that number which is equal to the days of the twelve months, and from infinite time comes the notion of infinite number.