Prometheus is spread on a rock, tied by chains, while a vulture feeds on his heart.  At first Prometheus endures it.  He endures it because the pain makes him real.  As long as the pain exists, so does he.  This makes his sacrifice significant, meaningful.

But the moment the vulture pauses, Prometheus has nothing more to resist.  Nevertheless, the force of resistance continues to push outward from his centre.  As the force encounters less and less pain, it pushes outward faster and faster.

Prometheus feels himself spreading into nothing, he feels himself being absorbed by the rock.  The hard core of Prometheus is dissolving and he begins to feel he is not himself but something else, a something that is forming out of his dissolution.

But there is no time for Prometheus himself to finish his thought.  The rates of expansion and dissolution have so increased that Prometheus disappears before he can articulate this doubt.

However, the doubt itself is all that is left of Prometheus and his magnificent sacrifice - eternal doubt on an eternal rock.