Nicene and Post-Nicene Fathers, Vol. IV:
Writings in Connection with the Manichæan Controversy.: Chapter 21

Chapter 21.—This Region of Light Must Be Material If It is Joined to the Region of Darkness.  The Shape of the Region of Darkness Joined to the Region of Light.

What, then, bordered on the side of the region which you call shining and sacred?  The region, you reply, of darkness.  Do you then allow this latter region to have been material?  Of course you must, since you asp. 139 sert that all bodies derive their origin from it.  How then is it that, dull and carnal as you are, you do not see that unless both regions were material, they could not have their sides joined to one another?  How could you ever be so blinded in mind as to say that only the region of darkness was material, and that the so-called region of light was immaterial and spiritual?  My good friends, let us open our eyes for once, and see, now that we are told of it, what is most obvious, that two regions cannot be joined at their sides unless both are material.

23.  Or if we are too dull and stupid to see this, let us hear whether the region of darkness too has one side, and is boundless in the other directions, like the region of light.  They do not hold this from fear of making it seem equal to God.  Accordingly they make it boundless in depth and in length; but upwards, above it, they maintain that there is an infinity of empty space.  And lest this region should appear to be a fraction equal in amount to half of that representing the region of light, they narrow it also on two sides.  As if, to give the simplest illustration, a piece of bread were made into four squares, three white and one black; then suppose the three white pieces joined as one, and conceive them as infinite upwards and downwards, and backwards in all directions:  this represents the Manichæan region of light.  Then conceive the black square infinite downwards and backwards, but with infinite emptiness above it:  this is their region of darkness.  But these are secrets which they disclose to very eager and anxious inquirers.