The Prince of Darkness. No matter what we call him, the Devil is our symbol for what is bad and undesirable. From our human perspective, we see the world as a struggle between light and dark. We want to vanquish the bad so the good can prevail. In fact, good and bad cannot be separated, just as you cannot separate a shadow from its source.
Darkness is simply the absence of light, and it is caused by errors that hide the truth.
Card 15 shows us these errors. First is ignorance - not knowing the truth and not realizing that we do not know. Second is materialism - the belief that there is nothing but the physical. As spiritual beings, we long for the Divine, but we lose contact with this source of truth if we trust only our senses. There is also hopelessness, which robs us of our joy and movement toward the light.