When John the Savage complains to Mond that the world feels so wrong, Mond argues that that's because John uses the old system of "right and wrong" as opposed to the new moral system of "happiness and unhappiness".
Balance Between Good and Evil: Mentioned to be part of the makeup of the world - no matter what sort of world the Planner decides to create, there will always be good and evil, happiness and unhappiness, even if the world is as close to a Utopia as possible.
Described In-Universe as an example: When the Savage gets into a discussion with Mond, Mond explains that the reason this world feels so wrong to the Savage is that he is still using the old system of good and evil, as opposed to the modern system of happiness and unhappiness.