13 Nor would the beings in this race of darkness have sought for food if it had not been sweet and pleasant, so that they would have died from want. For we find that all bodies have their peculiar wants, according to which food is either agreeable or offensive. If it is agreeable, it is said to be sweet or pleasant; if it is offensive, it is said to be bitter or sour, or in some way disagreeable. In human beings we find that one desires food which another dislikes, from a difference in constitution or habit or state of health.
Still more, animals of quite different make can find pleasure in food which is disagreeable to us. Why else should the goats feed so eagerly on the wild olives? This food is sweet to them, as in some sicknesses honey tastes bitter to us. To a thoughtful inquirer these things suggest the beauty of the arrangement in which each finds what suits it, and the greatness of the good which extends from the lowest to the highest, and from the material to the spiritual. As for the race of darkness, if an animal sprung from any element fed on what was produced by that element, doubtless the food must have been sweet from its appropriateness.
Again, if this animal had found food of another element, the want of appropriateness would have appeared in its offensiveness to the taste. Such offensiveness is called sourness, or bitterness, or disagreeableness, or something of the kind; or if its adverse nature is such as to destroy the harmony of the bodily constitution, and so take away life or reduce the strength, it is called poison, simply on account of this want of appropriateness, while it may nourish the kind of life to which it is appropriate.
So, if a hawk eat the bread which is our daily food, it dies; and we die if we eat hellebore, which cattle often feed on, and which may itself in a certain form be used as a medicine. If Faustus had known or thought of this, he would not have given poison and antidote as an example of the two natures of good and evil, as if God were the antidote and Hyle the poison. For the same thing, of one and the same nature, kills or cures, as it is used appropriately or inappropriately. In the Manichæan legends, their god might be said to have been poison to the race of darkness; for he so injured their bodies, that from being strong, they became utterly feeble. But then again, as the light was itself taken, and subjected to loss and injury, it may be said to have been poison to itself.
Source: Reply to Faustus the Manichaean (New Advent)