Why does Scripture say "one day the first day"? Before speaking to us of the second, the third, and the fourth days, would it not have been more natural to call that one the first which began the series? If it therefore says "one day," it is from a wish to determine the measure of day and night, and to combine the time that they contain. Now twenty-four hours fill up the space of one day-we mean of a day and of a night; and if, at the time of the solstices, they have not both an equal length, the time marked by Scripture does not the less circumscribe their duration. It is as though it said: twenty-four hours measure the space of a day, or that, in reality a day is the time that the heavens starting from one point take to return there. Thus, every time that, in the revolution of the sun, evening and morning occupy the world, their periodical succession never exceeds the space of one day. But must we believe in a mysterious reason for this? God who made the nature of time measured it out and determined it by intervals of days; and, wishing to give it a week as a measure, he ordered the week to revolve from period to period upon itself, to count the movement of time, forming the week of one day revolving seven times upon itself: a proper circle begins and ends with itself.
