• A belief in his own freedom. John Proctor does indeed believe in his own freedom and will do whatever it takes to protect it. Moreover, he will do whatever's necessary to protect the good name of his family. That's why he angrily recants his confession; he doesn't want future generations of Proctors to have to live with the stigma of being descended from a self-confessed witch.
  
  
• A supreme pride. This second characteristic is closely related to the first. John has such pride in the family name that he's willing to die for it. He knows that he will die, but what is more important for him is that his name will live on.
  
  
• A capacity for suffering. Like any tragic hero, John Proctor doesn't just suffer; he suffers greatly. Over the course of the play we find him suffering the consequences of his illicit affair with Abbie as well as being subjected to false accusations of witchcraft from his spurned lover. And yet through it all, John somehow manages to maintain his integrity. Now that's heroic.
  
  
• A sense of commitment. John shows considerable commitment to the cause of exposing the Salem witch trials as the absolute travesty of justice that they are. Very much a lone warrior for the truth, he shows considerable bravery in standing up to the absolute madness that has descended upon the town.
  
  
• Vigorous protest. As part of his commitment to the truth, John vigorously and publicly protests against the witch craze, even going so far as to challenge the validity of the proceedings in open court. It doesn't get him anywhere, of course, but it's a further sign of his status as a tragic hero.
  
  
• Transfiguration. If John hadn't already shown all the hallmarks of a tragic hero, then his being sent to the gallows would certainly have sealed the deal. John's execution on a trumped-up charge of witchcraft has secured his heroic status for the ages. Almost uniquely among the townsfolk of Salem, he stood up for the truth and paid for it with his life.
  
  
• Impact. John has made a considerable impact with his heroic life and even more heroic death. He's shown everyone that the Salem witch trials were based on nothing more than vindictive lies and scare stories. Although it'll be a while before everyone snaps out of their collective insanity, they'll eventually come to see that John was right all along and that they will need to take effective measures to ensure that such an almighty disaster never befalls the town again.

Tragedy is an imitation (mimesis) of an action that is serious, complete, and of a certain certain magnitude... through pity and fear effecting the proper purgation (catharsis) of these emotions. —Aristotle

The question of whether John Proctor, the protagonist of The Crucible, is a tragic hero (or whether the play qualifies as a tragedy) is debated in every classroom where it is taught. Cases can be made for either interpretation, but, using the standards of Aristotle, who first articulated the elements of tragedy, I would argue the Proctor is not a tragic hero.

Aristotle, of course, was writing about the Greek tragedies of Euripides, Sophocles, and Aeschylus, and his contribution was to pull out the key themes and create a blueprint in the Poetics. One of the key differences between the heroes of Greek (and Elizabethan) tragedy and that of Miller is that Proctor is a regular, unimpressive character; he is a simple farmer. Those from classical tragedy were kings, royalty, and gods. They were of high status—something that cannot be said of Proctor. Miller might argue that there is such thing as a tragedy of the common man (also see Death of a Salesman), but this is unheard of in the classical era. It is subjective if someone watching a play of film version of the play would find Proctor's mistake (his affair), his suffering, and his redemption cathartic.

The other key aspects of the tragic hero, again according to Aristotle, are:

1. A flaw/weakness or, as it is often taught in high school, a "fatal flaw." The Greek word is hamartia. If you were inclined to think of Proctor as a tragic hero, he is proud, which is a characteristic of many heroes.

2. The reversal of the character's fortunes, a sudden turn of events. In Greek, peripeteia.

3. The hero recognizes the reversal and that his flaw is partly responsible for his fate. A moment of clarity, if you will. In Greek, anagnorisis.

4. The character is guilty of excessive pride or hubris. He is humbled by circumstances and aware of it.

5. Finally, the punishment or consequences for the hero can be seen as excessive or, in the words of King Lear, the character is "more sinned against than sinning."

I think Proctor definitely embodies these characteristics of a tragic hero, but perhaps some of them more than others. Let's take them one at a time:

1) A belief in his own freedom: Proctor is something of a pariah in the village at times, because, although he is respected as a hard worker and honest man, he questions the effectiveness and character of the Reverend Parris, and as a result he does not attend church regularly. He does this because he does not think his spiritual dedication is dependent upon practicing his religion in the way expected by the community.

2) A supreme pride: Proctor takes pride in his own integrity, which is challenged when, in order to disprove the charges of witchcraft against himself and his wife, he must admit to his infidelity with Abigail.

3) Capacity for suffering: Proctor is willing to go to prison to fight for his life and that of his wife Elizabeth. He also suffers great anguish when he is forced to sign a false confession, which in the end he is unable to do.

4) A sense of commitment: once Proctor realizes the witchcraft accusations are tearing the village apart, he vows to do all he can to help fight for justice for the innocent.

5) Vigorous protest: Proctor is outraged by the court proceedings and the rumor-mongering of the villagers, and speaks out against the corruption and superstition he sees people falling prey to.

6) Transfiguration: Proctor's deep sense of personal pride is changed when he realizes it is more important to try and save Elizabeth and to help the village resist the witchcraft hysteria than to be right or to protect his own reputation.

7) Impact: Proctor's actions ultimately cause the end of the witch trials; his courtroom denouncement of Abigail Williams causes her to steal the Reverend Parris' money and leave town, confirming that she was manipulating the situation and was not actually "bewitched" after all.