Harper: In this world, there is a kind of painful progress. (5.10.2)
Harper pretty much sums up the progressive view of the play in this statement: moving forward isn't easy, but it's necessary. As hard as it is, we all need to be brave enough to change.
Prior: The world only spins forward. (Epilogue.31)
Here, at the end of the play, Prior echoes advice he received from Belize earlier on. He seems to be saying that not only is progress necessary, it's inevitable.