Aeschylus had represented as chained by Zeus to Caucasus, with
a vulture gnawing his liver, offered a perfect embodiment of
Shelley's favourite subject, "the image," to borrow the words
of his wife, "of one warring with the Evil Principle, oppressed
not only by it, but by all--even the good, who are 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." In the Greek
play, Zeus is an usurper in heaven who has supplanted an older
and milder dynasty of gods, and Prometheus, visited in his
punishment by the nymphs of ocean, knows a secret on which the
rule of Zeus depends. Shelley took over these features, and
grafted on them his own peculiar confidence in the ultimate
perfection of mankind. His Prometheus knows that Jupiter (the
Evil Principle) will some day be overthrown, though he does not
know when, and that he himself will then be released; and this
event is shown as actually taking place. It may be doubted
whether this treatment, while it allows the poet to describe
what the world will be like when freed from evil, does not
diminish the impressiveness of the suffering Titan; for if
Prometheus knows that a term is set to his punishment, his
defiance of the oppressor is easier, and, so far, less sublime.
However that may be, his opening cries of pain have much
romantic beauty:

"The crawling glaciers pierce me with the spears
of their moon-freezing crystals, the bright chains
Eat with their burning cold into my bones."

Mercury, Jupiter's messenger, is sent to offer him freedom if