In black you can read the word GOOD, in white the word EVIL (inside each black letter is a white letter). It's all very physiological too, because it visualize the concept that good can't exist without evil (or the absence of good is evil ).
Think of this poem called ‘The Eagle and the Wolf’:
There is a great battle
That rages inside me.
One side is a soaring eagle.
Everything the eagle stands for
Is good and true and beautiful.
It soars above the clouds.
Even though it dips down into the valleys,
It lays its eggs on the mountaintops.
The other side of me is a howling wolf.
And that raging, howling wolf
Represents the worst that is in me.
He eats upon my downfalls
And justifies himself by
his presence in the pack.
Who wins this great battle?
The one I feed