To answer this question we need to be aware of the subject of the poem. In this poem the speaker is admiring the startling energy of the Tiger that he sees and can only wonder whether it is demonic or godlike. Again and again, throughout the poem the speker asks the tiger who its creator was. The implied answer that is never actually given to us is that it can only be either God or the devil. Note how this questioning operates in stanza three:
And what shoulder, and what art,
Could twist the sinews of thy heart?
And when thy heart began to beat,
What dread hand? and what dread feet?
The focus is on the identity of the creator of the tiger, and the relentless questioning that goes on throughout the poem only serves to highlight the uncertainty of the speaker. Such an energy, such a force, could only be the creation of two people, and it is unsure which we can look to.