William Blake's "The Tyger," one of the most compelling poems in Songs of Innocence and Experience (1794), is an exploration of the nature of God and of good and evil. As your question notes, Blake's change of one word in the first and sixth stanzas—from could frame to dare frame—carries with it two central questions: What kind of being could make such a fearsome animal? And why would the same God who creates the lamb (innocence) also create something as fearful as a tiger (experience)? As Blake suggests, there is a frightening difference between what God is physically able to do and what he dares to do. Were you to see the illustration that appears with the poem, you would notice that Blake's tiger looks fairly harmless, more like a colorful and large domesticated cat than a predator. The poem, however, depicts a fearsome, almost mechanistic, animal that is created by a being whose intentions are ambiguous. In light of God's creation of a lamb, a symbol associated with Christ (the Lamb of God), and the tiger, an emblem of violence, one cannot easily determine either God's plan or his nature.
Blake advances the depiction of both tiger and its maker through a series of unanswered questions intended to cast doubt on the nature of the tiger's creator and of the tiger itself:
In what distant deeps or skies
Burnt the fire of thine eyes?
Blake begins to question the tiger's origin and creator. "Deeps" or "skies" may allude to Hell and Heaven or, in a more natural sense, the tiger might be a creature of the light or of the darkness. In either case, the reader is to understand that the tiger's origin cannot be determined. In the succeeding two lines, Blake uses allusions that make the tiger's creator a mystery:
On what wings dare he aspire?
What the hand dare seize the fire?
The first is most likely an allusion to Icarus, who aspires to fly but is brought down by Apollo, and the second is to Prometheus, who steals fire from the gods in order to help mankind and is punished by the gods. The reader is then left a bit puzzled by the nature of both animal and its creator. Is God omnipotent or, like Icarus and Prometheus, simply daring?
Blake's third and fourth stanzas make it clear that the tiger's nature is the product of mechanics rather than the breath of life, and its creator is, metaphorically, a blacksmith:
And what shoulder, and what art,
Could twist the sinews of thy heart. ...
What dread hand? and what dread feet?
The tiger's creator does not breathe life into the tiger; rather, using his strength and industrial skills (art), he is able to create the tiger's heart as if he were twisting cables or iron rods, as a blacksmith does. The reference to dread hand and dread feet alludes to the blacksmith's hands, wielding hammer and tongs, and his feet, which operate the bellows that keep the fire at the proper temperature. The industrial nature of this creation is even clearer in the following lines:
What the hammer? what the chain?
In what furnace was they brain?
What the anvil? ...
This depiction of the blacksmith clouds the nature of the God who makes the lamb and the tiger. In the creation of such a fearful creature, God becomes an industrial worker rather than a God who creates life by instilling life into a creation. The tiger, then, is an emblem of the evils of industrialization that Blake explores fully in his poem "London."
Perhaps more important, however, is the dual nature of God himself—"Did he who made the Lamb make thee?"—a being who is capable of creating both good and evil. In Blake's belief system, humanity is trapped in the tension created by the dual nature of God, who is not only capable of making an animal like the tiger but also willing to make such an animal. The God who makes the lamb is "meek & He is mild," but, in his role as creator of experience, he is a God who dares to "frame [the tiger's] fearful symmetry." Blake closes the gap between could and dares with an exploration of the duality inherent in lamb and tiger and of God's dual nature.