In order to work out the principal theme of a chapter, you first of all need to focus on its contents. This chapter of the novel focuses on a dream that Jonas has that obviously relates to his physical and sexual development, as in this dream he clearly indicates a sexual interest in his friend, Fiona. The way that this results in Jonas having to take medication to "treat" what is called the Stirrings in this society indicates that this chapter supports one of the overall themes of the novel, which is the way that so much of the lives of the citizens in this community are controlled and regulated to erase and eradicate any unpredictable emotions or feelings. What is significant is the way in which Jonas feels that he would like to experience the Stirrings again, even though he knows that this is something that is not permitted:
For a moment, though, he remembered the dream again. The dream had felt pleasurable. Though the feelings were confused, he thought that he had liked the feelings that his mother had called Stirrings. He remembered that upon waking, he had wanted to feel the Stirrings again.
Thus it is that we see the conflict of the individual vs. society being presented in this chapter, as we see yet another way in which the society has controlled its citizens to "protect" them from chaotic and turbulent sexual desires, whilst Jonas's desire to experience the Stirrings again clearly foreshadows his rejection of his community's laws and precepts.