by William Blake
The fire serves multiple purposes as an extended metaphor. First, it’s often associated with the Tyger, which contributes to the Tyger’s ferocity and sublimity (the fact it’s big, powerful, and mysterious). Fire is also a source of energy, and since the Tyger seems to be filled with fire, then he must also be filled with energy. In another sense, the fire of the smith’s furnace is the fire of creation, the means by which the Tyger was formed.