"Every wish immediately suggests its opposite. If I have a good thought, for example, I cannot help but have in me somewhere the opposite bad thought. In fact, it is a very basic point: In order to have a concept of good, you must have a concept of bad, just like you can't have up without down or black without white.
This idea came home to me when I was about eleven. I occasionally tried to help poor innocent woodland creatures who had been hurt in some way -- often, I'm afraid, killing them in the process. Once I tried to nurse a baby robin back to health. But when I picked it up, I was so struck by how light it was that the thought came to me that I could easily crush it in my hand. Mind you, I didn't like the idea, but it was undeniably there." - Carl Jung