To readers (and to Milo), it probably seems ridiculous to outlaw thinking altogether—after all, everyone is thinking all the time. But the Lethargians are actually talking about
active thinking here—they don’t consider daydreaming to be real thinking. The Lethargians are another example of humor in the novel, as their name derives from “lethargic,” or lazy. When Milo chooses to push back on the Lethargians, it again shows that he’s practical. But the Lethargians are also highlighting for Milo some of Milo’s faults, such as that he can’t pay attention for long periods of time.