This should be an easy concept for most people to grasp. William Blake summed it up pretty good with two of his poems, The Tyger and The Lamb. The real crux of these two poems is in The Tyger, the lines:
What immortal hand or eye
Could frame thy fearful symmetry?

The fearful symmetry is the fact that both evil and good need each other. This is because, in defining good, you need to reference evil, and vice versa. The narrator of The Tyger is a voice of experience, though, and is therefore tainted by the real world. In order to fully grasp the concept, one needs to also read The Lamb, which will provide a counter point. Draw your own conclusions.