In Chapter 16 of Harper Lee's To Kill a Mockingbird, the morning after the mob scene, Atticus attempts to appease Scout and Jem by explaining that he wasn't in any true danger.
One thing he explains is that "[a] mob's always made up of people, no matter what." Atticus's view is that people always have their good traits and their bad traits. A human being's best traits are his/her abilities to reason and to empathize with others. Hence, in saying the above, Atticus is saying that even members of a mob still have the ability to reason and to empathize.
Atticus further stresses his point that even a mob is made up of rational minds by asserting the following:
So it took an eight-year-old child to bring 'em to their senses, didn't it? ... That proves something--that a gang of wild animals can be stopped, simply because they're still human. (Ch. 16)
In speaking of a child bringing the members of the mob to their senses, he is referring to the fact that Scout is the one who broke up the mob by reminding Walter Cunningham of his humanity. She does so inadvertently by asking him to say "hey" for her to his son Walter Cunningham Jr, who is in her class at school. She further inadvertently reminds him of his humanity by pleasantly conversing with him about his entailment, saying, "Entailments are bad," because she remembers Atticus's advice that it was the "polite thing to talk to people about what they were interested in." She further reminds him that Atticus once promised Mr. Cunningham they would "ride" out his entailment together. It is all of these inadvertent gestures of kindness, politeness, and empathy that bring Mr. Cunningham to his senses. He suddenly remembers just how much respect he has for Atticus, squats down, puts his hands on Scout's shoulders, promises to tell Walter Jr. she said "hey," and tells the rest of the mob to clear out.
The fact that Mr. Cunningham comes to his senses is proof that, as Atticus phrases it, "Mr. Cunningham's basically a good man ... he just has his blind spots along with the rest of us." Hence, Mr. Cunningham's behavior serves as proof for Atticus's optimistic view that a mob is still made up of human beings, and human beings will always have the ability to think rationally and to empathize.