In Harper Lee's To Kill a Mockingbird, Jem particularly begins to form opinions about fairness after Tom Robinson's trial.
Jem was able to see that the jury's guilty verdict was unfair since all evidence, especially Robinson's crippled physique, pointed to his innocence, not his guilt. Jem was so upset by the jury's verdict that he cried "angry tears" and repeatedly muttered, "It ain't right" on the way home from the courthouse, which is clear evidence he is beginning to form his own opinions about what is fair and just.
The next day, during a conversation with Atticus, Jem raises questions about the fairness of awarding the death sentence for a rape case, "even if he was guilty," and the fairness of the jury system (Ch. 23). Jem points out that the jury could have sentenced Robinson with twenty years in prison and hints at the unfairness of sentencing a "colored man" to death just because he is colored. Jem even states, "We oughta do away with juries," since it was the jury who convicted Robinson and sentenced him to death based on nothing but the testimonies of the victim and her father (Ch. 23). All of these statements show us that Jem is questioning what his society currently deems as fair and forming his own opinions about what is truly fair.