How is the amount of Light received by the Moon always the same ?

And how is it that from eternity to eternity this Ship of Light is filled uniformly and receives neither more nor less ? But this contrivance was not a wise one. For it would be right that, at the time when the Refining is great, the Moon should receive more, that is to say, instead of being filled till the time of full moon, it would be right that she should be filled in five days. For if their statement were true, it would be right that what I have said should be the case. [Mani's teaching has not aided the Refining.] For to-day there is much of Mani's Teaching, and so it is clear there is also much Refining! But as a hundred years ago, this Teaching did not exist—would that it did not now—it is evident that the Refining of Light a hundred years ago would be less than it is to-day. And if the Refining of Light was not the same in amount then and now, how was the Moon then and now uniformly filled till full moon ? And when the Refiners were few in number, there was not less Light for the Moon, nor to-day when the Manichaeans abound is there any Light added to it. But when there were no Manichaeans, and when they are now in existence, there is no increase in the Moon to-day though they exist, just as there was no lessening in the Moon when they did not exist. [P.18, l.31.] So by the Moon, fixed in the Height which they have made as a mirror for themselves, it is possible for that secret falsehood of theirs to be brought to light. For if the existence and non-existence of the Manichaeans are alike to the Moon, the lying Teaching is refuted by what is peculiarly its own, in that its existence is on a level with its non-existence. And if they do not exist for the Moon, for which they imagine they do now exist in a very special way, they do not in a very special way exist for God the Lord of the [P. 19.] Moon. Thus from the Luminaries they receive a special refutation who imagine that they are recognised by the Luminaries. And, in fact, does not the reasoning of arithmeticians4 convince them that when those who persuade are many, much more do those that receive measure out; and when there are many floods the rivers are filled above their limits and rise beyond their wont ? [xxxviii]