20 You say, in reply, that you believe in what Manichæus has not proved, because he has so clearly proved the existence of two natures, good and evil, in this world. But here is the very source of your unhappy delusion; for as in the Gospels, so in the world, your idea of what is evil is derived entirely from the effect on your senses of such disagreeable things as serpents, fire, poison, and so on; and the only good you know of is what has an agreeable effect on your senses, as pleasant flavors, and sweet smells, and sunlight, and whatever else recommends itself strongly to your eyes, or your nostrils, or your palate, or any other organ of sensation. But had you begun with looking on the book of nature as the production of the Creator of all, and had you believed that your own finite understanding might be at fault wherever anything seemed to be amiss, instead of venturing to find fault with the works of God, you would not have been led into these impious follies and blasphemous fancies with which, in your ignorance of what evil really is, you heap all evils upon God.
Source: Reply to Faustus the Manichaean (New Advent)