Weeks later, it becomes clear that Karenin’s cancer is spreading, but he still goes to work every day with Tereza. Human goodness, the narrator says, if it is truly pure, can only exist if the recipient of said goodness has no power. Thus, humankind’s “true moral test” is the mercy one shows towards animals.
Karenin has no power and is completely at Tereza’s mercy. Tereza technically has nothing to gain by showing Karenin mercy—he has nothing to give her anymore—but she does so anyway because she is innately good. Kundera thus argues that true goodness can’t exist if there is something to be gained by it.