And the subject he loved best to dwell on was the image of One warring with the Evil Principle, oppressed not only by it, but by all—­even the good, who were deluded into considering evil a necessary portion of humanity; a victim full of fortitude and hope and the spirit of triumph emanating from a reliance in the ultimate omnipotence of Good.  Such he had depicted in his last poem, when he made Laon the enemy and the victim of tyrants.  He now took a more idealized image of the same subject.  He followed certain classical authorities in figuring Saturn as the good principle, Jupiter the usurping evil one, and Prometheus as the regenerator, who, unable to bring mankind back to primitive innocence, used knowledge as a weapon to defeat evil, by leading mankind, beyond the state wherein they are sinless through ignorance, to that in which they are virtuous through wisdom.  Jupiter punished the temerity of the Titan by chaining him to a rock of Caucasus, and causing a vulture to devour his still-renewed heart.  There was a prophecy afloat in heaven portending the fall of Jove, the secret of averting which was known only to Prometheus; and the god offered freedom from torture on condition of its being communicated to him.  According to the mythological story, this referred to the offspring of Thetis, who was destined to be greater than his father.  Prometheus at last bought pardon for his crime of enriching mankind with his gifts, by revealing the prophecy.  Hercules killed the vulture, and set him free; and Thetis was married to Peleus, the father of Achilles.

Shelley adapted the catastrophe of this story to his peculiar views.  The son greater than his father, born of the nuptials of Jupiter and Thetis, was to dethrone Evil, and bring back a happier reign than that of Saturn.  Prometheus defies the power of his enemy, and endures centuries of torture; till the hour arrives when Jove, blind to the real event, but darkly guessing that some great good to himself will flow, espouses Thetis.  At the moment, the Primal Power of the world drives him from his usurped throne, and Strength, in the person of Hercules, liberates Humanity, typified in Prometheus, from the tortures generated by evil done or suffered.  Asia, one of the Oceanides, is the wife of Prometheus—­she was, according to other mythological interpretations, the same as Venus and Nature.  When the benefactor of mankind is liberated, Nature resumes the beauty of her prime, and is united to her husband, the emblem of the human race, in perfect and happy union.  In the Fourth Act, the Poet gives further scope to his imagination, and idealizes the forms of creation—­such as we know them, instead of such as they appeared to the Greeks.  Maternal Earth, the mighty parent, is superseded by the Spirit of the Earth, the guide of our planet through the realms of sky; while his fair and weaker companion and attendant, the Spirit of the Moon, receives bliss from the annihilation of Evil in the superior sphere.