The fool, fixed in his folly, may think
He can turn the wheel on which he turns.
These lines come in the midst of a scene highly reliant upon the Devil's temptation of Jesus in the Gospels. Here four different tempters come to Becket with different types of temptations that vary in subtlety from obvious physical temptation to the temptation of power. The tempter here seeks to remind Becket of his previous life of "mirth" and love, a life of pleasure, before he became archbishop of Canterbury. These lines are Becket's reply. The point is that one cannot return--literally turn back the wheel. What is done is done. If one seeks one's old life, one will find that this time around, things are different. He suggests that similar things happen from generation to generation because people fail to learn from others' mistakes, but in one's own life, one cannot repeat, one cannot return to one's youth. It is an attitude of folly. The last tempter, however, is much more subtle. He seeks to tempt Becket into doing good, but good that seeks spiritual power from beyond the grave as a martyr, as one is idealized and, in fact, idolized. It is a power more powerful than a mere king, who dies and is replaced. It is an ongoing power. It is the temptation, in fact, that Becket acknowledges that he is most inclined toward, but he finally resists, saying:
The last temptation is the greatest treason:
To do the right deed for the wrong reason.
One does good because it is good. Or, perhaps, one does good for God's glory and not for amassing spiritual power, even beyond the grave as a saint or martyr. (A true saint does not seek to be one.)