For Harry and his friends' third year at Hogwarts, their challenge shifts from fighting incarnations of the dark lord Voldemort to instead taking on Sirius Black, a man who's believed by the entire wizarding world to have been Voldemort's right-hand man. After spending twelve years in the wizard prison Azkaban, where he was serving a life sentence for brutally murdering twelve innocent muggles and his best friend, Peter Pettigrew, Black escapes in the months before the school year begins, supposedly so that he can find and kill Harry. Though Black's crime and his imprisonment in Azkaban take center stage, it's by no means the trio's only engagement with the wizarding justice system. By offering the reader and Harry multiple ways to engage with the system, the novel is able to offer a more complete picture of how it functions. Most importantly, Harry's experiences with justice in Prisoner of Azkaban suggest that the wizarding justice system isn't actually just at all--instead, it's corrupt and more concerned with making a show of punishing supposed criminals than with administering fair trials, uncovering actual criminals, or offering a reliable system that doles out predictable and sensible punishments for crimes.
Until Harry learns the truth in the Shrieking Shack, the events surrounding Black's imprisonment appear to make for a simple open and shut case. Twelve years ago, Black supposedly betrayed Lily and James Potter by revealing their whereabouts to Voldemort, which led to their deaths. Then, in an event that makes Black seem even more maniacal, days later he murdered twelve innocent muggles as well as one of his best friends, Peter Pettigrew. For these crimes, he was given a life sentence in Azkaban. During the time that these events are treated as facts, it appears as though Black got what he deserved. This sets up Harry's initial understanding of the wizarding justice system, at least when it comes to hardened criminals like Black, as reasonably fair and just: Harry is led to believe that if a person murders innocent people and cooperates with Voldemort, they will naturally receive punishment from a government that's positioned as being on the side of good.
Though Harry doesn't question anything in regards to Black until the end of the novel, it's worth noting that this perception of justice doesn't square with his personal dealings with magical law enforcement. When the house elf Dobby used magic in the Dursleys' house in Harry's second year, Harry was blamed and threatened with expulsion if he used magic outside of school again--yet, when Harry uses magic both accidentally and on purpose during Aunt Marge's visit, Cornelius Fudge, the Minister of Magic, laughs it off and insists that Harry has nothing to worry about. This then becomes Harry's first indication that the wizarding justice system isn't something he can rely on to treat accused individuals fairly or with impartiality--for whatever reason, Harry is too valuable to punish in his third year when he wasn't a year before. It's also telling that there's no indication that Black ever went on trial for his crimes; he was sent to Azkaban immediately. In other words, he never got the chance to convince anyone of his own innocence--something that, in a more functional system, might have given him a chance at freedom.
Because Harry's emotions and his personal stakes in his own brushes with the law color his ability to ask difficult questions about how the justice system works (see Storytelling and Perspective theme), the only way he's able to see that the wizarding justice system isn't actually just is when it attempts to wrongfully convict the hippogriff Buckbeak. After Buckbeak hurts Draco Malfoy during a lesson (in retaliation for something that Malfoy was told explicitly not to do), Lucius Malfoy registers a complaint against the hippogriff with the Committee for the Disposal of Dangerous Creatures, which results in a string of trials and appeals. Harry, Hermione, and Ron all know that what happened to Malfoy wasn't Buckbeak's fault--Hagrid told his class outright that hippogriffs are proud creatures and will behave violently if insulted--so they initially throw themselves into helping Hagrid build his case to prove Buckbeak's innocence to the Committee. After the first trial, however, Harry and the trio learn that the Committee isn't as impartial as they'd like to think it is: Lucius Malfoy has power over the Committee, which means the Committee is far more interested in pleasing Mr. Malfoy than it is in dealing with facts in Buckbeak's case. This results in them sentencing Buckbeak to death, despite evidence, witness testimony, and legal precedent that all suggest the hippogriff is being wrongfully punished for a natural and known quality of his species, not because he did anything wrong.
Especially once it comes to light that Sirius Black isn't actually guilty--Peter Pettigrew betrayed the Potters, faked his own death, and framed Sirius in the process--all of this suggests that the wizarding justice system is more concerned with creating a show of justice than it is with actually finding out the truth. This is supported most poignantly by what Hagrid tells Harry about the dementors, the non-human guards of Azkaban: the dementors don't care at all about who's guilty and who's innocent; they care only about having bodies, souls, and happiness around to feed on. They will attack and suck the soul out of anybody, from true criminals to innocent children. The dementors then come to symbolize the entire system of justice as espoused by the Ministry of Magic, which proves itself willing to punish or threaten anyone if given the chance.
Harry's choice to believe Sirius's version of events suggests that after his dealings with these aspects of wizarding law enforcement and the court system, he's developed a healthy suspicion of its ability to actually uphold law and order, conduct fair and unbiased trials, or even consider the possibility that seemingly simple court cases might be more complex than they appear at first glance. Instead, Harry comes away with the sense that justice in his world can only happen when individuals listen and try to learn the entire story of a given case--and in the cases of Sirius and Buckbeak, justice can only be served when individuals take matters into their own hands, rather than relying on a system that favors performances designed to create the illusion of government.
Justice Quotes in Harry Potter and the Prisoner of Azkaban
Non-magic people (more commonly known as Muggles) were particularly afraid of magic in medieval times, but not very good at recognizing it. On the rare occasion that they did catch a real witch or wizard, burning had no effect whatsoever.
But this didn't tally at all with Harry's past dealings with the Ministry of Magic. "Last year, I got an official warning just because a house-elf smashed a pudding in my uncle's house!" he told Fudge, frowning. "The Ministry of Magic said I'd be expelled from Hogwarts if there was any more magic there!"
"The guards told Fudge that Black's been talking in his sleep for a while now. Always the same words: 'He's at Hogwarts...he's at Hogwarts.' Black is deranged, Molly, and he wants Harry's dead. If you ask me, he thinks murdering Harry will bring You-Know-Who back to power. Black lost everything the night Harry stopped You-Know-Who, and he's had twelve years alone in Azkaban to brood on that."
"We're witnesses," said Harry. "You said hippogriffs attack if you insult them. It's Malfoy's problem that he wasn't listening. We'll tell Dumbledore what really happened."
"Yeah, don't worry, Hagrid, we'll back you up," said Ron.
"Didn't they want to help, sir?" said Percy.
"Oh yes," said Dumbledore coldly. "But I'm afraid no dementor will cross the threshold of this castle while I am headmaster."
"But you were innocent!" said Hermione.
"Think that matters to them? They don' care. Long as they've got a couple o' hundred humans stuck there with 'em, so they can leech all the happiness out of 'em, they don' give a damn who's guilty an' who's not."
Hermione burst into tears. Before Harry could say or do anything, she tucked the enormous book under her arm, and, still sobbing, ran toward the staircase to the girls' dormitories and out of sight.
"Can't you give her a break?" Harry asked Ron quietly.
"No," said Ron flatly. "If she just acted like she was sorry--but she'll never admit she's wrong, Hermione. She's still acting like Scabbers has gone on vacation or something."
"Malfoy's dad's frightened the Committee into it," said Hermione, wiping her eyes. "You know what he's like. They're a bunch of doddery old fools, and they were scared. There'll be an appeal, though, there always is. Only I can't see any hope....Nothing will have changed."
"But if--if there was a mistake--"
"KEEP QUIET, YOU STUPID GIRL!" Snape shouted, looking suddenly quite deranged. "DON'T TALK ABOUT WHAT YOU DON'T UNDERSTAND!"
"Of course," Lupin breathed. "So simple...so brilliant...he cut if off himself?"
"Just before he transformed," said Black. "When I cornered him, he yelled for the whole street to hear that I'd betrayed Lily and James. Then, before I could curse him, he blew apart the street with the wand behind his back, killed everyone within twenty feet of himself--and sped down into the sewer with the other rats..."
"Yes, I do," said Dumbledore quietly. "But I have no power to make other men see the truth, or to overrule the Minister of Magic..."
Harry stared up into the grave face and felt as though the ground beneath him were falling sharply away. He had grown used to the idea that Dumbledore could solve anything. He had expected Dumbledore to pull some amazing solution out of thin air. But no...their last hope was gone."
"It didn't make any difference," said Harry bitterly. "Pettigrew got away."
"Didn't make any difference?" said Dumbledore quietly. "It made all the difference in the world, Harry. You helped uncover the truth. You saved an innocent man from a terrible fate."