In Frankenstein, Mary Shelley uses the recurring symbol of fire, called a motif, to illuminate both the dangers of too much knowledge as well as the dual nature of humanity. Human beings are capable of both great goodness as well as great evil. Her use of the symbol, then, illuminates two universal themes that apply, broadly, to both knowledge as well as human nature.
Victor Frankenstein, for example, tells his new friend, Captain Walton,
Learn from me, if not by my precepts, at least by my example, how dangerous is the acquirement of knowledge and how much happier that man is who believes his native town to be the world, than he who aspires to become greater than his nature will allow.
Victor feels that he has been destroyed by the knowledge he gained when he figured out how to create life. He feels that, like the tree he saw struck by lightning when he was a child, he has been destroyed by what he has learned. He compares himself to a "blasted tree" and feels that his knowledge is actually responsible for his destruction. Further, characters are frequently described as having a sparkling eye—character such as Victor's mother as well as Felix DeLacey—a description that uses light as a symbol of life and happiness. The creature's eye, however, is described as "dull yellow," and he is created by the light of a "half-extinguished light," as if to signify his moral ambiguity. Just as he is capable of great good, such as when he helps the DeLacey family in their poverty, he is also capable of evil, such as when he murders William Frankenstein. Early in the creature's life, he says,
I found a fire which had been left by some wandering beggars, and was overcome with delight at the warmth I experienced from it. In my joy I thrust my hand into the live embers, but quickly drew it out again with a cry of pain. How strange, I thought, that the same cause should produce such opposite effects!
The fire brings him pleasure as well as pain, and he, himself, is capable of bestowing pleasure and pain on others through his actions. Shelley, then, uses the recurring symbol of fire to illuminate several global themes.