What is the central idea of the poem the tyger by William Blake?

The poem "The Tyger" explores the nature of God it ponders His act of creation and delves into the nature of evil in the Word.

http://www.sparknotes.com/poetry/blake/section6.rhtml

Also, it reflects about a human's desire to know about God...if He truly exists, and the reason why He made this world like it is now.