In the famous Zen koan often referred to as “Joshu’s Dog,” a monk asks the Zen master Joshu, “Does a dog have Buddha nature?” to which Joshu replies “Mu,” or “no thing.” Mu is represented in the Chinese language as a negative character, but as Zen master Taisen Deshimaru explains, “it [mu] is not a negative idea. Mu is not relative to the fact of existing; it's nothing. It is very difficult to explain. What is mu? Nothing and everything....Mu does not exist. Mu exists, but without noumenon. A great koan.”
This well-known koan offers an interesting ontological paradox that applies to our conceptions of sin in the Christian West, which is, “how can something exist without actually existing?” Few would deny that there is evil in the world, and yet its existence is affirmed only by phenomenon, the trail of tangible disorder that sin leaves in its wake. In the words of Deshimaru, sin exists, “but without noumenon.”
Julian of Norwich, the 14th century anchoress, said that "sin is nothing." How is this so? Like a parasite, sin feeds off goodness. It twists and deforms the good, but it would not exist without it, just as "darkness" would not exist without "light." The assumption is dualistic, equating "light" with "goodness," as the book of Genesis paints it.
Sin is not some monster roaming around on its own (though one would think otherwise after reading St. Paul). Rather, it is the monster we create that cannot live without us. We birth it, give it life, by our actions, and subsequently allow it to feed off of us our entire lives, deforming our good nature and marring our world. Sin is very real, and yet it does not really exist. Understand? Mu.