What new philosophy motivates Hamlet in Act 5?
Hamlet has been so concerned through the first four acts of play about making good choices and the potential consequences of all of his actions that he has been slow to take action against Claudius and avenge his father's murder. But in Act 5 he realizes that he doesn't have control over everything in his life and he needs to just respond to the moment. He specifically says to Horatio that he realizes that "there is a divinity that shapes our ends rough hew them how we will." What he means is that we humans can make choices and take actions that give shape to our lives, but that there is a fate/destiny/Providence that is also in play that will also give shape to our lives -- probably some of the finer points of our lives are out of our immediate control. Hamlet follows this thought up later when Horatio is cautioning