In Act Four scene four of Hamlet, by William Shakespeare, Hamlet talks to one of Fortinbras's captains as his army crosses Denmark on their way to Poland. The captain explains that they are going to fight for a piece of land he would not, for anything, even want to own.
After the captain leaves, Hamlet ponders this act and compares Fortinbras's willingness to marshal an army and lead them some distance in order to capture (and die for, if necessary) a worthless piece of land ("even for an egg-shell") to his own consistent unwillingness to take action despite his greater reason to do so.
Hamlet feels as if everything he sees and hears seems to be aimed at spurring his "dull revenge." He goes on to compare himself to a beast because he has done nothing "but...sleep and feed" but "no more." He knows, however, that man was created to do more than that; God gave him "god-like reason" which should not go unused. Whatever has caused his inaction, Hamlet now determines to act, since he has the mind and "strength of will" to do so.
O, from this time forth,
This new sense of purpose and motivation, which he has had several times before but to no effect, does seem to be the final prompt Hamlet needs to finally take some action. Shortly after this proclamation he escapes captivity, consigns his former friends to death, and goes back to Denmark where he eventually does avenge his father's death.