The lunar month of 29½ days opposes Mani's view about the Moon.

And why, indeed, is there a Moon for twenty-nine days and a half ? Let the false Teaching which disguises itself offer a proof on this point by means of a natural demonstration. But let us strip it that it may appear bare without any truth. Let them tell us, therefore, concerning this part of a day why it is defective and not completed ; is there no superfluous Light in any of the months, so that the deficiency for this day may be filled up ? But when it (i.e., the day) is defective it is not finished, and if there is superfluous Light (?) it is not completed. And if on account of the small amount of Light that day is imperfect, there would [P. 20.] be a chance that other days too would be imperfect. And in like manner when the Light increased, it would be right that the days should be found increasing as well. The shortage of Light, however, does not make any lessening in the Moon, nor does the increase of Light fill up this defective part. So let this defective part of a day convict the Heretics that they are altogether lacking in truth.