We first learn about how Jonas feels about lying in chapter 9 on page 68, when Jonas reads his rules. It is the last rule, the shortest, and the only one that is not a procedural instruction. This indicates that it will be significant.
Jonas is shocked to read the last rule. After he “steeled himself” to read the rule, he reflects that “he had been trained since earliest childhood, since his earliest learning of language, never to lie” (70). Significantly, lying is associated with imprecise speech rather than morality. In fact, “the reason for precision of language was to ensure that unintentional lies were never uttered” (70-71). Preventing lying is deeply engrained in the community’s culture.
The thought of lying to anyone makes him very uncomfortable, and when he considers that others might also lie it is “terrifying” (71). Jonas considers asking other adults if they lie, but realizes that if he did, “he would have no way of knowing if the answer he received was true” (71). The fact that everyone tells the truth is part of the certainty the community is trying to ensure. Uncertainty makes people uncomfortable, and preventing discomfort is paramount.
Since openness about feelings is so essential to the community's emotional control (thus the nightly ritual involving feelings), this rule signifies Jonas's separation from community life. One of the reasons that Jonas reacts so strongly to the rule is that he realizes that it makes him very different from everyone else. He has never had anything to hide before, so he has had no reason to lie. That rule, coupled with other ominous warnings about pain, foreshadow difficulties in Jonas's future and the continual separation of Jonas from his family and friends.