Harper Lee shows the theme of finding the truth through knowledge and experience in different ways. The most obvious way is by telling the entire story from the point of view of an adult who once was a child at the time that Maycomb enters the period of tension and debate caused by the trial of Tom Robinson.
To Kill a Mockingbird is narrated by an adult Scout Finch. However, the details of the events are described the way that Scout would have seen them unraveling back when she was eight years old.
While Harper Lee cleverly uses the details as they would be described from the perspective of little Scout, she still contrasts AND combines those details with the insight that the adult Scout adds to the narrative as a mature narrator who is now looking back and analyzing those events in retrospection.
The adult Scout is able to tell the difference between what she thought she knew and what she actually knows now. This diverse focus coming from the same character adds depth to the narrative as well and embodies the theme of maturity as something that can only be learned through experience and exposure.
Think about this: Had young Scout never witnessed the events that took place in Maycomb, it is very unlikely that the adult Scout would have ever made the connections and realized the truths about social dissension, racism, and class discrimination that were going on in her city for centuries. Everyone else seems oblivious to it, but Scout was someone who was meant to be different and know better. After all, she was the daughter of an extremely smart man.
And how does Scout show this?
Out of all the characters in the novel, the one we have more evidence about changing is Scout. After all, it is she the one telling us the story about how she learned so much in this dark period of time. These are some a few of the huge lessons experienced by Scout:
a) She learns from Calpurnia that all people should be treated with respect, "even a Cunningham". That one's table is to be shared and one's guest is to be treated hospitably. Begrudgingly at first, Scout learns to be civil to others.
b) In chapter 16, Scout learns about humanity by appealing to that of Mr. Cunningham's the night that his lynch mob went after Tom Robinson. Even Atticus comments how an eight year old has the capacity of instilling common sense into grown adult change.
... it took an eight-year-old child to bring 'em to their senses.
c) In chapter 19, it was Scout who assured Dill (another dynamic character who was very affected by Tom Robinson's trial) that his crying about the mean way the prosecutor insulted Tom was acceptable; that no one deserves to be talk to like that, no matter what color they are.
d) Scout is even able to document the changes taking place as Jem grows into a young man. His attitude, his hunger, his frustration at the system after the trial. She understands how injustice affects people differently. She is also able to tell the difference between someone being unjustly prejudiced (like her aunt Alexandra) versus dignified and proud (like Calpurnia).
Therefore, Harper Lee uses Scout as the mouthpiece of how the important truths in life must be learned through experience. The Robinson case, her father Atticus, her caregiver Calpurnia, and the experiences with Boo Radley (not to mention the murder attempt at the hands of Bob Ewell) were tough reality checks that made the adult Scout look back and re-tell the story under a new, different, and much more knowledgeable point of view. Scout would have not learned to be any more than yet another biased citizen of Maycomb unless she had had those experiences under her belt.