What is the theme of the poem "The Tyger" by William Blake?

William Blake's poem "The Tyger," written much like a metaphysical conceit, has as its theme the mysteries of God's creations. 

It is a God who is inscrutable to man that has created such a being as a tiger, for in man's limited knowledge, God is all-good. Thus, in the awareness that his knowledge is limited, the speaker wonders in a series of rhetorical questions about the mysteries of good and evil. For instance, he asks the tiger,

Did he smile his work to see?
Did he who made the Lamb make thee?

What immortal hand or eye
Could frame thy fearful symmetry?

Perhaps, it is only man who has defined good and evil in the context of what he knows. Or, is evil, perhaps, named only by man so that he can recognize good in its contrast since his powers of cognition are not that of the Creator's? Clearly, Blake's poem demonstrates his belief that man must witness, examine, and resolve the apparent paradoxes of life. Critic Alfred Kazin writes of Blake,

In "The Tyger," he presents a poem of triumphant human
awareness, and a hymn to pure being.

Additional Source: 

Kazin, Alfred. "Introduction". The Portable Blake. The Viking Portable Library.